Technical Tips > Post 84
1986 C4 MAF Sensor replacement?
Vettech:
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.
peterw:
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
Vettech:
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:
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.
peterw:
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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