NSW Corvettes
Technical Tips => Post 84 => Topic started by: peterw on February 15, 2020, 01:12:15 AM
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My 86 (with 50,000 miles up) is throwing a MAF error code (34).
I have checked the connection and it is OK, can't find anything obviously wrong anywhere else so have been doing research on a replacement.
So far I have only found one place in US that has an original (new) MAF and it is $1200.
Many places offer rebuilt ones (as does a place in Victoria).
Most have non genuine replacements. Has anyone used one of these and could recommend it or any other course of action?
I want the car to be reliable. It is bone stock.
Thanks
Peter
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Hello Pete.
R U referring to 6E3-A-44 and page 45. In the Service Manual ?
You need to follow the process, else very expensive.
I'm not SURE - but I would guess the MAF off other GM cars may he compatable.
There is not much in a MAF. Basically a small cct and a Hot Wire.
You have 2x Earths, 12 Volts power from the MAF Power Relay, 5 volts Signal from the ECM..
And a burn off signal from the Burn Off Relay.
The cct powers up and the wire is heated, as air flows into V engine it cools the wire, the 5volt signal is modulated from 0 ~ 5v.
At 5v it is high for high air flow and ECM applies more fuel. At idle it may be 0.6v so ECM sets enough fuel for idle.
Often a car with a faulty MAF has a rough or stumbling idle.
Any wiring with O/C or HR's - esp cct 998 Dk Green to Maf connector "C" will cause issues.
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And just to finish.
The burn off is activated "JUST AFTER" the engine is switched off (if certain criteria met - run time/engine to temp)
The ECM applies via the burn off relay etc a full 12volts to the wire, lights it up, and burns any crap off the wire.
So - any openings, cracks in the duct from filter to MAF, will allow "muck" into the engine, which can cook onto the Wire causing incorrect sensing ergo 0~5v signal ergo incorrect fuel mix ergo dicky idle or running. (Mostly idle)
The two relays MAF Burn off and MAF power can after years (30yrs) of use develope dicky contacts, change both as a matter of course. Sorry I'm having internet issues which keeps dumping me off line.
What I'm getting at here is, if the burn-off does not or has not occured for some time then the Wire could be in a really crappy state, this will play havoc with the ECM's ability to metre the correct fuel to air ratios for efficient engine running
Do you get code 33 as well ? Let me know and if a cct diag required.
P.S In the past I've had 33,34 &36, all these relays after conversion are up on the LH side fire wall, covered in all manner of crap, but more importantly are very close to manifold heat, I've had relays cooked, plugs cooked, wiring hanging on by one minscule strand. Best of luck.
Regards.
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Thanks for the detailed response.
Good tip about the relays. I realise that any 35 year old electrics are likely to fail as the resin hardens and cracks in the components so would replace any that could be a potential failure.
Now, rebuilt MAF from Victoria (Injectronics or alternative?) or aftermarket one from USA?
I'm not 100% sure the MAF is faulty so I will youtube it and see if I can test it somehow.
Pete
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I just pulled the MAF, it's Bosch so is likely the original.
It looks pristine, no corrosion or muck anywhere.
Still could be faulty though.
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You should be able to see the Hot Wire.
Now when I say covered in crap - that crap in most cases is microscopic and is layered.
Run thru the tests, including the test for burn off.
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I cleaned the MAF and it made a marginal improvement. It still won't start without the throttle but if I start using the throttle I can get it to idle (just).
Yesterday I tried the MAF out of my friend's C4 and there was no improvement.
I tried my MAF in his car and it ran perfectly.
I am now suspecting the MAF Relays and want to replace them.
My research tells me
10094701 - AC Delco 212-305 - Relay, MAF burnoff
10067925 - AC Delco 212-300 - Relay, MAF power
There is lots of info about these relays but it often seems contradictory.
The US forum say the relays are different, Corvette Central only list the one for both jobs.
The ones Rock Auto have don't look correct to me.
Ecklers list an incorrect OE part number (I don't trust them anyway).
Has anyone found a local source for these relays or could point mt a reliable source in USA?
Thanks
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You can try Ian at Chevparts at Yennora.
Or
Dave at Ponti World (USGM Service parts) down at Albion Pk.
And U R correct, there is a lot said in forums, books etc about the relays looking the same but being different.
The 87 GM Service Manual cct diag shows.
Burn off Relay has 4 connections
D & F are Coil c/w parralell resistor across coil.
A & E are one Make Contact (ie. NO Contact).
MAF Power relay has 5 connections
D & F same as above.
A & E & C IS A CHANGEOVER contactor.
The 3rd Relay often confused is the Fuel Pump relay - looks the same as the previous two.
And a quick glance at the diagram they look identicle internally, untill you look closely at the pins.
B & C ARE THE COIL & resister connection. And
The CHANGEOVER Contactor is A & E aaaannd D. So very much different internally.
And Why!? Only the "person in GM" who wrote the spec or GOD knows why.
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Sorry Pete - I've just reread your last post.
So if started without foot on throttle - it wont start and run - yes ?
Could be a few things.
1.
If some one fiddled with throttle body - the Idle Air screw could be misadjusted.
Follow book to adjust - from memory 0.54v dc on a metre.
2.
These TPI V8's have a 9th injector, "COLD START INJECTOR" controlled independantly when certain criteria met (# starts, time, temp)
But at first start from cold should run for up to 8 sec.
And from your valve jockying (parts swapping), the faults definitely in your car but I doubt its the MAF.
The MAF should not effect cold start - yes maybe rough idle.
If suspect CS Injector check,
CINJ/CLU Fuze 5A in Aux fuse block.
Then cold start injector - from memory not easy to get at.
Then ThermoTime Switch - basically a switch with thermal bi-metal strip and heater wire.
This switch is normally closed when cold, powered up when in "start" and CS inj fires fuel in, and heater heats up (~8sec) - once at temp, the switch opens and cuts off the CS inj. Upon second start, since the switch is hot or at manifold temp, it may operate for 0 thru 8sec, if at operating temp, the switch cuts the CS inj out of cct, since its now no longer required. CS inj located LH side under fuel rail near Injector 5 from vague memory.
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In the past it would start and idle high for a couple of seconds then settle down, I expected this was the ECM doing it's job.
Now if I just turn the key it will fire but not keep running.
If I pump the throttle a little it will start and run then die if take foot off throttle (till it warms up).
I doubt anyone has fiddled with it, I have had it for 2 years and it has run perfectly, nobody works on it except me. I am hesitant to adjust the idle, the symptom might go away but the problem would remain.
I should have mentioned that since cleaning the MAF it now has error 33 (instead of 34 beforehand). There is a subtle difference between 33 and 34 and I'm not sure what it is.
I will definitely check the CS injector, that sounds a real possibility.
Pete
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These TPI V8's have a 9th injector, "COLD START INJECTOR" controlled independantly when certain criteria met (# starts, time, temp)
But at first start from cold should run for up to 8 sec.
I will definitely check the CS injector, that sounds a real possibility.
Pete
Have you ever considered getting a different tune on your prom ? I had an updated tune with a cold start circuit in it put on a chip from the USA by a guy that works on TPI for a living,I still have the 9th injector but nothing is attached to it ( plus I have no vats,no egr and a few other bits changed,It just uncomplicates the car somewhat ) )
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Have you ever considered getting a different tune on your prom ? I had an updated tune with a cold start circuit in it put on a chip from the USA by a guy that works on TPI for a living,I still have the 9th injector but nothing is attached to it ( plus I have no vats,no egr and a few other bits changed,It just uncomplicates the car somewhat ) )
Playing a tune on my prong?
Seriously, can the ECU be reprogrammed?
That would surprise me as 1984 is early days of computerised engine management.
I am a carbie and points guy so I am learning this as I go.
The car is bog standard and I not keen to modify it (unless I have to or unless I get a big benefit from it). I realise the science of engine management would have come a long way since 84 so am open to it.
This would get me out of my depth so I would need to find someone to tune and maintain the system.
Pete
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The answer to your question is yes it is old and yes it can be tuned, well the removable proms(chip can be reprogrammed )
Mine is only a couple of years newer than yours and I had mime all sorted out and it has not given me any problems since about 2013
the alterations I have on mine are
Fans on at 180
Trans drops in to overdrive at 60 kph instead of 80kph
Later model tune to get rid of the 9th injector
complete vats removal from the prom
Egr removed at the prom
Obviously a custom tune is out of the question as your car would have to go to the USA to be tuned, burt a new prom with tune improvements can be made in the USA and sent to you for under $200 (I am guessing as mine was )
The person I found to burn me a new tune is very well known on Thirdgen.org (a great f body site) and he has since opened up a tuning business in the USA
You can find him on Facebook as he posts on this page
T.P.I. Tuned Port Injection
His name is Brian Van Schoiack and he is very helpful,He may also be able to offer some insight into your current problem seeing as he knows the system like the back of his hand
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The answer to your question is yes it is old and yes it can be tuned, well the removable proms(chip can be reprogrammed )
Mine is only a couple of years newer than yours and I had mime all sorted out and it has not given me any problems since about 2013
the alterations I have on mine are
Fans on at 180
Trans drops in to overdrive at 60 kph instead of 80kph
Later model tune to get rid of the 9th injector
complete vats removal from the prom
Egr removed at the prom
Obviously a custom tune is out of the question as your car would have to go to the USA to be tuned, burt a new prom with tune improvements can be made in the USA and sent to you for under $200 (I am guessing as mine was )
The person I found to burn me a new tune is very well known on Thirdgen.org (a great f body site) and he has since opened up a tuning business in the USA
You can find him on Facebook as he posts on this page
T.P.I. Tuned Port Injection
His name is Brian Van Schoiack and he is very helpful,He may also be able to offer some insight into your current problem seeing as he knows the system like the back of his hand
excellent information should be a great help
Bfit
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https://tunedperformance.org/?fbclid=IwAR2aHYqM-ZTmywTpzgPqqq5jEs_IKKa8LbkDU2UbB-o0hBeymzx90TXbtJI
This is Brian's website If it makes life easier,I would suggest anyone that has a C4 get a recalibrated memcal from him,It is easily the best thing you will do for your car
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Fans on at 180
Trans drops in to overdrive at 60 kph instead of 80kph
Later model tune to get rid of the 9th injector
complete vats removal from the prom
Egr removed at the prom
Thanks for that info.
My car obviously has a fault which I'd like to solve first but having a custom chip is a great idea. I like the idea of including improvements that were done in later cars.
The radiator fans on my C4 come on at 220 deg. Not sure what the thermostat is.
This is correct for the model but seems really high, does anyone know why it is so high? Maybe an emissions thing?
If the ECM tells the fan to come on at 180 then the fans would run almost all the time unless the thermostat was changed to be less than 180.
I would worry that running the car at 180 when it is designed to run 220 might cause an issue.
Has the 180 fan switching changed how your car runs?
Pete
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Pete
I am very happy with the way my car runs
Changing the tstat is not such a hard thing to do either,One other thing you should consider is removing the hose from your throttle body,it is something we do not need in Australia and again it is an easy bypass
Maybe email Brian and start the ball rolling and ask him about your problem and ask him about the chip
Is your car rhd or lhd ? if rhd I would be looking at the earths to start with
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My car is LHD, manual, unmodified. It has 50,000 miles on it.
It was in a collection in California for many years and did about 1000 miles per year in the last 25 years. It would have spent a lot of time sitting which isn't good for it.
I expect it to have some niggles which I can hopefully overcome with the help of the forum.
Pete
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If suspect CS Injector check,
CINJ/CLU Fuze 5A in Aux fuse block.
Then cold start injector - from memory not easy to get at.
Then ThermoTime Switch - basically a switch with thermal bi-metal strip and heater wire.
This switch is normally closed when cold, powered up when in "start" and CS inj fires fuel in, and heater heats up (~8sec) - once at temp, the switch opens and cuts off the CS inj. Upon second start, since the switch is hot or at manifold temp, it may operate for 0 thru 8sec, if at operating temp, the switch cuts the CS inj out of cct, since its now no longer required. CS inj located LH side under fuel rail near Injector 5 from vague memory.
Is the Aux fuse block the one behind the dash?
If I have to pull the dash apart is there anything else related to the intake system in there?
Pete
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Yes. Sorry - ive had battery and other issues.
It is behind the display panel in the middle of the dash.
Along with Starter enable relay, audio alarm module, hazard flasher and horn relay.
With an Ohm and Voltmeter you should be able to firstly check continuity of thermal resistor and actual injector/switch to earth.
Then between cct 806 to earth, turn on ignition - @ 12 volts, if no 12v - fuse gone.
If no continuity thru injector or thermal switch and heater coil - you know what to replace
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One thing with Vats removal is the ability to remove that start enable relay as even with vats removed from the memcal the key still has to talk to that relay for the car to start
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The only issue I've had with keys - is the actual tweezer type contacts which fail to contact the chip on the key.
Replace or retension - no more issues - or read the chip resistance and hard wire an equivalent in parrallel with the contacts - this does defeat the purpose of the security function - I like the car to be where I left it, upon my return...
Now FYI .
The CS inj is near number 5 injector - but injects direct into the manifold.
The Thermo Time Switch is located under the Throttle body on the front of the engine, looks like a Temperature Sensor, in fact, to its right is a smaller sensor which is in fact Coolant Temp Sensor.
I'd check these out before ripping the dash apart.
Regards.
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Vettech
I just ran the test you gave me in post 5 and wasn't able to get a result because my car had been giving me code 33 beforehand.
I assume it works when you are getting code 34 (which mine originally did).
It will idle when warm but hunts around and still gives me code 33.
That page out of the manual said something about a rough idle test so I really need a shop manual.
What manual do you have? It looks to be very informative.
The Official Chev manual is expensive at $240. Haynes for $67.
I have looked at online/disk manuals and can't tell if they are any good.
Any recommendations from you guys?
Pete
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I have the GM Service Manual ST-364-87 AND
the GM Electrical Diagnosis Service Manual Supplement ST-364-87 EDM.
They are a strange setup, the GMSM has basic electrical, all the Code stuff and Six fold out double sided six panel circuit diagrams - concise but a bitch to read and wire trace thru, with failing eye sight.
The GMEDM is in chapters covering each system, circuits, pics, large diagrams, text on tests and usefull info for that system, also mine is coloured, but only because I went over the cct lines with coloured pencils. The cct lines have cct #'s and colour written on, but its easier when it is Coloured.
These are for '87. I also have Haynes for 87 & 78 somewhere - not a patch on the GM books, pics etc are dark and hard to read, waste of time but they came with the cars.
I bought the GM's from Ian at Chev Parts at Yennora, cant remember how much, but have been well worth while. I also bought when I first got the 78 a Builders Book, this is not avail for C4's, but shows exploded diagrams of just about everything and would be the Bible for anyone in NCRS, since it shows every detail for every nut, bolt and washer. This is great when your C3 has been butchered - sorry I mean converted, and nearly everything behind the dash has been ripped out and scrapped.
If you intend to keep the car and do your own work - the GM books are well worth while. I replaced a rear hub using the GMSM.
Regards.
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If cct 998 goes O/C, then the ECM will see a high signal voltage and set a code 33, meaning it thinks the MAF air flow is too high.
On my 87 I got 33, 34 & 36, if not one then the other, the relays look the same BUT THEY ARE NOT.
Ensure correct relays, then I ripped out some wiring or cut & replaced connectors, all the connectors, wires etc were well & truely cooked, hardened and filled with greace and oil and other general crap. Eventually all went well.
Sorry if I'm a bit vague, with all this Corvid19 going on, its hard to concentrate.
Regards
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Thanks for the info.
I will contact Chevparts about the manual.
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Once I got out and driving again the car seemed to come good. Maybe the computer had to learn again after I reset it.
No MAF code and car seemed to run a lot better, wouldn't idle when cold though.
Symptoms and research told me it was likely Idle Air Control valve though I didn't get a code pop up.
I have replaced the IAC and now car starts properly, idles when cold and seems 100%.
Car has only done 50,000 miles in 34 years so these kind of gremlins don't surprise me.
Thanks to you guys and youtube I am back on the road.
Pete
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My 87 is now starting but rolls over like it has a big cam am suspecting the idle control valve or the wiring connections the schematic shows 2 coils so I am tipping if there is a high resistance connection in plug or wiring it could upset the balance operation of the valve. I found a test M/F test plug to check voltages but was $90 US plus post so will do some looking tomorrow. My car also had a 33 code which i cleaned and checked all connections and code dissapeared.
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The faulty IAC symptoms were - no idle when cold and warm idle was 1100 and sometimes hunted. Otherwise the car went well. My googling told me this was typical of IAC problem.
You will need to take the throttle body off to get to the IAC. It looks accessible but they didn't allow any room to put a spanner on it. I tried to make a spanner for it but failed.
When I was pulling it apart one of the plastic vacuum tubes broke. I tried bending a few of them and they were very brittle.
I replaced them all with rubber vacuum line (4mm) and just required a couple of bits of plastic tube to adapt to existing fittings.
It is possible that a broken vacuum tube was to partly to blame for the idle problem but the IAC was very grungy up so I was happy to have changed it.
It's not hard to change out the vacuum hoses once the throttle body is off.
You will need a gasket for the throttle body, I couldn't find one in Australia so I made one. It's time consuming making gaskets from 1.6mm gasket sheet so if you are ordering from USA you might get one of them thrown in. Cost about $US10-15 for a complete kit.
I ordered my IAC from Summit Racing on a Tuesday and it arrived on the Friday. Amazing.
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Checked the voltages on the 2 coils in IAC had 9.13V on green pair & 0.9V on blue pair with ignition on, engine not running.Also thru tested wiring direct from computer to check for continuity, coils on IAC both test at about 56 ohms. Removed vacuum pipes from rear of manifold & connected to vacuum pump both held good vacuum so no vacuum leaks in hoses & valves,Fuel regulator. Starting to run out of ideas.
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Finnaly back onto Forum.
How did all this go on your C4 Peter's.
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Checked the voltages on the 2 coils in IAC had 9.13V on green pair & 0.9V on blue pair with ignition on, engine not running.Also thru tested wiring direct from computer to check for continuity, coils on IAC both test at about 56 ohms. Removed vacuum pipes from rear of manifold & connected to vacuum pump both held good vacuum so no vacuum leaks in hoses & valves,Fuel regulator. Starting to run out of ideas.
My research tells me the MAF, IAC and TPS are what controls the engine at low revs.
I was wondering if you tried cleaning the MAF. For the cost of a can of MAF cleaner it could be worth a shot.
TPS is very easy to test as well, mine varied from 1V to 4.2V which is what the youtube gurus said it should.
I was lucky in having a friend with an 86 and could do a MAF swap to test it. It's very easy to take out so if your 87 has the same MAF as mine you are welcome to do a swap with mine and test yours. I am at Peakhurst so not far away.
Pete
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I know your reply is for Sirfixalot but.
When mine played up some time ago, I think I had carby cleaner, thus it was cleaned.
But touching that hot wire did not appeal.
But I had varying codes, as stated early, new (correct) relays, new connectors, new wires.
Was running great until the coolant decided to depart recently.
The thing wrong with this, is I don't have the smoking gun but a mass of probables.
This bit is for you MY081 :banana: :nutkick:
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I'll send some more tomorow, I just typed for an hour only to loose it all.
But we need some more details about how the ECM operates - esp after looking at 8A-20 - 0.
Geoff - if your getting a constant voltage on one coil all the time, I think this is in-correct
There are three possible scenerios.
1. The IAC is only operated when SOC is reached (SOC - Standard Operation Conditions)
2. The cct is only energized when at idle and adjustment required.
3. Adjustments like Drive engaged, A/C applied etc.
4. Alternatively it may give constant pulsing voltages to either Hi or Low coil.
But for a constant voltage on one coil, means the ECM is trying to Drive Hi or Low constantly.
Note that 8A-20-0 may not be correct, I would imagine the "A" coil (blue wires) should be marked Hi and the other coil both wires Low.