1986 C4 MAF Sensor replacement?

Ausrs · February 27, 2020, 04:44:55 PM

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

Vettech · February 27, 2020, 05:04:35 PM

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.

peterw · March 24, 2020, 06:23:31 PM

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

Vettech · March 25, 2020, 08:14:40 PM

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.

Vettech · March 25, 2020, 08:48:20 PM

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

peterw · March 26, 2020, 12:07:14 AM

Thanks for the info.

I will contact Chevparts about the manual.

peterw · July 08, 2020, 10:01:16 AM

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

sirfixalot · July 08, 2020, 07:51:52 PM

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.

peterw · July 09, 2020, 12:47:40 AM

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.

sirfixalot · July 09, 2020, 07:00:37 PM

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.

Vettech · July 20, 2020, 02:35:42 PM

Finnaly back onto Forum.

How did all this go on your C4 Peter's.

peterw · July 20, 2020, 03:21:42 PM

Quote from: sirfixalot on July 09, 2020, 07:00:37 PM
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

Vettech · July 20, 2020, 08:16:56 PM

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

Vettech · July 20, 2020, 09:53:53 PM

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.