Guru diagnostic help needed!!!(please)
Thread Starter
Instructor
Joined: Dec 2004
Posts: 105
Likes: 0
Guru diagnostic help needed!!!(please)
Allright guys I dont know what to do, I posted about this three times but still havent found the problem. After hours of my own diagnostics and 2 mechanics (one had it for 2 months, but not going to charge me). Heres the symptoms that happened from one day to the next. Car idles erratically, gas smell is unbearable and the car dies if not accelerated. This happens at cold start up or warm doesnt matter. For some reason when I dissconnect the MAF the car runs decent. The second mechanic (vette shop) has tested or replaced every thing he can think of (MAF, ECM, MAF burn off module, fuses, sensors etc..) Nothing is changing the behavior. He also opened up the dash and checked all the computer wiring for shorts. Any suggestions? Its a 85 with no mods.
Thread Starter
Instructor
Joined: Dec 2004
Posts: 105
Likes: 0
its the stock 350 tpi. I dont think the problem is fuel related because by disconnecting the MAF the car runs better. I'm sure the mechanic checked it. He seems to thinks there is some sort of a short he cant find.
Race Director
Joined: Feb 2004
Posts: 12,518
Likes: 19
If the car runs better with the mass air disconnected, I would bet a ten dollar bill you have a bad maf sensor, bad relay or bad ecm. Swap the relay. @ $7 it is about the only part on the car I believe is worth swapping, just for the piece of mind. Borrow a good maf sensor from a friend or fellow vette owner and see if it helps.
I just re-read your origional post. If the mechanic simply swapped out the maf sensor for a remanufactured unit, I would not be shocked if the replacement is bad. Most companies that reman them do a poor job. Not unheard of to have 3 or 4 in a row test bad out of the box. Also open up the harness and inspect the maf connector. They are known to break the wires internally, just behind the weatherpack. I strongly recommend trying to find a known good used one and borrow it for a test fit.
I just re-read your origional post. If the mechanic simply swapped out the maf sensor for a remanufactured unit, I would not be shocked if the replacement is bad. Most companies that reman them do a poor job. Not unheard of to have 3 or 4 in a row test bad out of the box. Also open up the harness and inspect the maf connector. They are known to break the wires internally, just behind the weatherpack. I strongly recommend trying to find a known good used one and borrow it for a test fit.
Last edited by Pete K; Aug 30, 2005 at 06:05 PM.
Burning Brakes
Joined: Aug 2002
Posts: 926
Likes: 2
From: chandler az
My guess from the symptoms is that you have an injector that is partially stuck open. Disconnecting the MAF causes the computer to change modes(I think - limp home mode) but I am not sure.
Also pull the vacuum line off of the fuel pressure regulator and smell it - if the diaphram breaks raw fuel will be sucked into the vacuum line.
Also pull the vacuum line off of the fuel pressure regulator and smell it - if the diaphram breaks raw fuel will be sucked into the vacuum line.
Thread Starter
Instructor
Joined: Dec 2004
Posts: 105
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I already took it to the vette shop and hes ready to give up he said he tryed or tested everything he can think of. I dont know what to do, I think I am going to go pick it up on Friday, does anyone have a 85 specific diagram outling the maf wiring route? or is their any vette places you guys recommend in or around los angeles?
p.s. its not throwing any codes
p.s. its not throwing any codes
Last edited by batoutofhell; Aug 30, 2005 at 06:56 PM.
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Team Owner
Joined: Jun 2005
Posts: 46,295
Likes: 2,596
From: Northern, VA
St. Jude Donor '15
"In honor of jpee"
you list so little about yourself except finally mentioning that you're in los angeles. do you think it would help a bit if you mentioned what section or community of los angeles you are located in? and possibly, what "vette shop" you've been taking it to? 
some people may be unwilling to have to pry out the info from you---in this case, more info is better.
some people may be unwilling to have to pry out the info from you---in this case, more info is better.
Drifting
Joined: Mar 2005
Posts: 1,923
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From: GA
Originally Posted by Pete K
I believe the maf system on 85's are unique. Something about a seperate burn off module. Hopefully this thread will keep bumping up to the top, and someone in this pickle already can help shed some light.
Right now it seems you are to a "Hail Mary" with it not throwing any codes.
Some deductive reasoning here.... Changed ECM, no help. Changed MAF's, no help. Disconncet MAF and it runs better. All wiring checked, no problems found. MAS module changed, no help.
The 2 things that can be causes of something like this and haven't been changed are; Ignition module and PROM chip. The 1985 PROM chips were known to be bad from the factory and had a service bulliten out on them. If yours was never changed, that'd be the first thing I'd do.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Does the car have the correct injectors? Put a fuel pressure gauge on the end of the fuel rail, pressurize the rail and turn the ignition off. Fuel pressure should not lose more than 1 psi in an hour according to GM or you may have a leaky fuel injector. Fuel smell is a symptom of too rich mixture. One thing I would do is to find the latest GM part number for the ECM and buy it from GM. My 87 went through 3 ECM's a year apart and then the last one had a new part number and has lasted 10 years. I think that early ECM's had no buffering from engine sensors and voltage spikes induced from the engine compartment damaged the ECM's. The new part number I think has buffered inputs that can survive spikes.
What part of L.A.?
Drifting
Joined: Mar 2005
Posts: 1,923
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From: GA
Originally Posted by jfb
My 87 went through 3 ECM's a year apart and then the last one had a new part number and has lasted 10 years. I think that early ECM's had no buffering from engine sensors and voltage spikes induced from the engine compartment damaged the ECM's. The new part number I think has buffered inputs that can survive spikes.
Advanced
Joined: Nov 2004
Posts: 77
Likes: 0
From: Athens Alabama
It sounds like you have tried most of the sensors and electronics but have you checked the mechanical items? Have you done a compression check on the cylinders? Also, pull off your valve covers and do a very careful check for any broken valve springs(sometimes they're hard to see). I had similar symptoms on my 93 LT1 and I replaced and cursed all of the sensors and electronics when it turned out to be 2 broken valve springs.
Racer
Joined: Feb 2002
Posts: 293
Likes: 0
From: Benoni Gauteng
I would really go with checking the fuel. The MAF sensor tells the ecm how much air is going in the engine, and adjusts the fuel accordingly. That could be why you see a diffrence when you disconnect it. A friend of mine had a similar problem and all signs pointed to the maf sensor. It ended up that the fuel pump was faulty
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Well, the gas smell is obviously from to much fuel entering the motor-I'd have your mechanic or a friend do the leak down test on the injectors, unless I missed it-that's the one thing he hasn't checked!
Drifting
Joined: Feb 2002
Posts: 1,952
Likes: 2
From: Little Rock Arkansas
May also want to consider a vacuum leak as a possible cause. VERY common on L98 TPI engines and cause symptoms exactly like what you're describing (including the fuel smell if it's bad enough).
-Jeb
-Jeb
