Author Topic: 1986 C4 MAF Sensor replacement?  (Read 1960 times)

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Offline peterw

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1986 C4 MAF Sensor replacement?
« on: February 15, 2020, 01:12:15 AM »
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

Offline Vettech

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Re: 1986 C4 MAF Sensor replacement?
« Reply #1 on: February 15, 2020, 04:42:29 PM »
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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Offline Vettech

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Re: 1986 C4 MAF Sensor replacement?
« Reply #2 on: February 15, 2020, 05:14:58 PM »
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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Offline peterw

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Re: 1986 C4 MAF Sensor replacement?
« Reply #3 on: February 15, 2020, 06:19:11 PM »
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

Offline peterw

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Re: 1986 C4 MAF Sensor replacement?
« Reply #4 on: February 15, 2020, 06:31:10 PM »
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.

Offline Vettech

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Re: 1986 C4 MAF Sensor replacement?
« Reply #5 on: February 16, 2020, 10:22:40 AM »
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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Offline peterw

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Re: 1986 C4 MAF Sensor replacement?
« Reply #6 on: February 22, 2020, 12:48:37 PM »
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



Offline Vettech

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Re: 1986 C4 MAF Sensor replacement?
« Reply #7 on: February 22, 2020, 04:03:46 PM »
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.
Vettech.

Offline Vettech

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Re: 1986 C4 MAF Sensor replacement?
« Reply #8 on: February 22, 2020, 08:36:12 PM »
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.

Vettech.

Offline peterw

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Re: 1986 C4 MAF Sensor replacement?
« Reply #9 on: February 22, 2020, 10:51:25 PM »
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

Offline Ausrs

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Re: 1986 C4 MAF Sensor replacement?
« Reply #10 on: February 23, 2020, 02:55:16 PM »





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 ) )

Offline peterw

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Re: 1986 C4 MAF Sensor replacement?
« Reply #11 on: February 24, 2020, 01:29:33 AM »
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