Help! mass air burn off module on 85

[money pit]

a year ago i bought 85 corvette with blown mot0r thought i got good deal had motor rebuit and instaled
1 st mechanic gave up after 8 months 2 mechanic gave up 2mounths now have noware to go because
i am sick of puting parts in it never needed 3 new mass air,1 new burn off module $7oo.oo WTF,1new ECM ,many relays,top end all new dash board apart trying to trace it back ,1 mechanic says might
instrament cluster what do u think?

takes 5 min be for i can drive then sum times when i put it to the floor it goes then cuts out then goes again
mass air coded, burn off not coming through burn off module 12volt goes in but not out when u shut car off
help please!!!!

 

[boomdriver]

How many of these mechanics were Corvette mechanics?

I ask, because a real Vette mechanic would NOT be throwing parts at a car. He'd be following the FSM step by step guide to troubleshoot ANYTHING that could possible be wrong...

3 MAFs? seriously?

I feel for ya...there is NOTHING cheap or INEXPENSIVE about a Corvette....especially a 25 yr old Corvette. The biggest mistake everyone makes is assuming that because it IS 25 yrs old that it IS/WAS yesterdays technology and they treat it like it was a carbed motor from 1969...its NOT. The technology is everybit as complicated as a brand new car....in fact, the new ones control systems are based on what you have on your car..just the leatest greatest version, thats all.

Get a FSM and a REAL vette mechanic and I'm sure it'll be fine. Until you can pin down one thing instead of just naming off a bunch of things that never worked, I know I can;t help much.

Gimme some symptoms. I get that you're tired of it.

Has ANYONE done the simplest things? like.........

fuel pressure
fuel filter
fuel pump/screen
ign timing & module
valve adjustment
exhaust & cats clear
fuel injectors

Theres LOTS that can be done to solve this cars problems IF you know what to look for and where to start.

Noid lite
fuel pressure gauge
timing lite
FSM and reading glasses.

 

[money pit]

thank u for your thoughts they are well known mechanics but not vette mechanics niether am i what is
FSM

 

[money pit]

Mass air

Yes all the things u mentioned have been done and more every part will be new if i keep going and i will be broke

 

[money pit]

not the first person to sugest that, would of been great if i did it $7000.00 ago
does anyone know a site were i can get factory service manual

 

[boomdriver]

not the first person to sugest that, would of been great if i did it $7000.00 ago
does anyone know a site were i can get factory service manual

do a search here for FSM and there is a link for CD versions or books.
EBay has books for sale, used or new. 75 to $150 depends on which.

Not trying to be adversarial, but I have to believe that this is not impossible. Its a machine. Thats all. If you understand the machine then you understand the problems it can have.
Someone has overlooked or neglected to do something simple. If your techs are not trained in GM or Corvette technology then they are NOT qualified. Corvette utilizes systems that no other car used in the 80s when the C4 matured. Only a top shelf Caddy had similar eng mangt systems. Todays mechanics will tend to approach a C4 and treat it like it was a carbed 77 Camero and try to adjust things that were never meant to be touched. They disregard sensors and their value to the system. This engine has 4 different temp sensors...actually 5 if you count air temp sensors as well as coolant for the engine. MORE for cabin and outside air. Knowing which does what and for which system is crutial. Some are switches. Some are variable resistance value sensors. Dirty connections on the eng coolant temp sensor can cause all form of hell and misfire. A stuck EGR time/temp switch will make the eng burp and backfire. A harness thats allowing cross-signals will defeat all sensor inputs. All, easy to track down and figure out IF you have the FSM and some patience.

I just do not accept the fact that experienced mechanics have thrown that much money at that motor and it still coughs and won;t run. That tells me they're GUESSING. Like I said, I'm not trying to offend, I'm trying to get you pointed in the right direction so you can enjoy the finest car made in this country and one of the finest in the world. It should never be a headache and a regretable choice.

Its pretty basic...it gets fuel and it needs air. The air is metered and fuel is adjusted accordingly. The prom dictates the operating parameters and the prom IS the tune, not the old school adjustment of a few things. getting the timing in the 'window' allows the ECM to do the rest...it'll run as long as its close.

This could easily be the dist being off by 1 tooth. Happens everyday. Wires crossed. Wires pinched inside the dist cap.

If its still showing MAF codes then there is a problem with the MAF system. Wiring harness, relays, plugs and especially grounds. Your new FSM will tell you where the 20 or so ground contacts on the frame live. There are 5 that are CRITICAL by the oil filter.

Somebody missed something. its not witchcraft. These things are predicatble but only when good, factual info is available. Try this or adjust that don;t work. Whats to adjust? the ECM does that. The correct sensors and the correct data must be available to the ECM or you end up with a mess like this. HAs anyone put it on a scan to see what the ECM is seeing?

If I were there I';d do it for you because I'm positive that its nothing unusual. It never is. Fuses, links, bad harness wire...bad weatherpak connectors. All can be traced if you understand the system. being a good mechanic is fine for hub bearings (which you have'nt explored yet) or changing u-joints but when it comes to the engine performance, it takes someone experienced with C4 electronics to figure it out.
Get your FSM and study the basics, then start with the flow charts and follow them exactly. There is NOTHING that is not covered in the GM FSM.
Try to find an older copy because the newer book on the electrical is in B/W I'm told. The older books are color exactly as the harness is. Much easier to follow.

Good luck

 

[money pit]

thanks for all your thoughts boomdriver
i read something on another thread last night that the gauge fuse runs the burn off and egr but if blown gauges still work .so i checked
the fuse and wouldant u know it it was blown. egr wires were arcing u would think story over nope .yes mass air code went away.
now have no code but car still does not run to full potential when u punch it will cut out then go sometimes will cut out a few time sometimes not
by the way if i drive like a normal person the car is all most perfect would like to modify if i can car to run right

 

[John Robinson]

Wires

By now every wire connector and wireing harness has been pulled tugged bent moved etc. My guess is you don't have sensor problems as much as you have wiring problems. For some reason the wires get brittle and break easily along with the connectors. Since the ECM only reads the output of the sensors (The voltage going to ground) through the ground wires any bad wire will then send the ECM into a frenzy trying to compensate for the bad signal. The same holds true if the wire going to the sensor is broken. When the car is idling shake the wires to the TPS or any of the sensors and see if the engine changes how it is running. Unless you are able to use a voltmeter and check the wiring harness for shorts and breaks in the wires it might be faster and cheaper to get a new wiring harness for the engine compartment.

Like boomdriver said this isn't rocket science. Beleive it or not I had a BMW with almost the same type of engine mangement system. After I went through the wires one at a time and repaired them it ran like new. So when my corvette started to act the same way(Almost as you describe how yours is running) I went to the wiring harness and the grounds first. I found bad wires from the TPS the MAF the water pump temp sensor. Remade the wires back to the common ground bundle in the harness and it now runs like it should.

Here are the locations of the grounds.

 

[money pit]

can a bad instrament cluster cause perfomance ploblems

 

[John Robinson]

can a bad instrament cluster cause perfomance ploblems

I don't know what you are saying is bad with yours but lets go back to the beginning. If the grounds are bad then anything that is connected to them will not function. These cars are all about grounds. Forget about the idea that if you have 12 volts going to something and it does not work then it must be bad. The ECM only cares about what the ground voltage is. That tells it how to adjust the air, fuel, timing etc. If the signals get bad enough to the ECM from a sensor it will go into limp home mode. Or if it does not get the proper sequence of information it may stay in closed loop. The first thing to do is get out of the box that all off us have been in with these cars they are not your lawnmower. Lets take for instance the TPS (I will not try to be technical) What is it and what does it do. It is a potentiometer that has 5 volts coming into it from the hot wire. in closed throttle position it allows about .04 to.05 volts to the ground. As the throttle moves to wot that voltage will increase to about 4.5 to 5 volts. going to the ground wire. What does that mean to the ECM. It simply is telling it how open the throttle or butterfly is and adjusts the fuel flow accordingly (remember the accelerator pump on a carb) So if you do not have a good connection to it then again the ECM goes crazy and starts to adjust the fuel, air , timing (sort of like when we fiddled with the idle screw, or reset the float height or set the timing on our cars of the 50s and 60s)but it does it in milliseconds and does not get it's hand dirty.

 

[TedC]

TPS should be set at .54 volts at idle +/- .08 volts. Setting on the higher range makes the engine feel more responsive. I find the stock setting at .54V works best. WOT TPS voltage should exceed 4.4 volts opening the throttle plates by hand and confirm using the gas pedal.

 

[John Robinson]

TPS should be set at .54 volts at idle +/- .08 volts. Setting on the higher range makes the engine feel more responsive. I find the stock setting at .54V works best. WOT TPS voltage should exceed 4.4 volts opening the throttle plates by hand and confirm using the gas pedal.

Perhaps your right but as I stated in my reply I was using that as an example of how these cars are ground signal orientated to the ecm. If you go by the FSM for my 93 it says the range is .5 to 5 volts. I was old school in my thinking until I repaired that BMW motorcycle which had an ECM and was a ground signal system like the corvette. In that case it only took me a month to realize that and from then on I was able to fix the wiring harness were it was burned by a short. My next challenge was the corvette when it started to backfire and not run above 3000RPM. that fix was repairing the broken wires in side of the TPS connector and then replacing the ground wires from the water pump temp sensor the map sensor, and then cleaning the ground bundle in the wiring harness were they and others come together. The final thing was to clean the frame grounds and the grounds behind the battery and on the bell housing. After that it ran like a corvette. I don't know for sure if that is what the problem with the op's car is but I have noticed that I am not the only who has repaired there cars by carefully checking that the wiring is in good shape. I think that we all tend to forget that it has a life cycle just like any other part on the car but we tend to just pull the connectors and reconnect them without any thought to the condition they are in and. Then the frustration starts over who has replaced the most parts at the greatest cost.

 

[K & B Special Products]

can a bad instrament cluster cause perfomance ploblems

Short answer, no. You can remove cluster/IP & car will function normally.

 

[money pit]

thank you all for your thoughts.
i will get FSM and check every ground
i know the instrument cluster needs to be rebuilt some times it goes blank and the directinal and high beam lights a guy that rebuilds them said
that was one of the symtems that is y i asked that ?.

has any one ever have a ploblem with with remand mass air parts?
once again thanks

mechanics said they checked all grounds but they did not have FSM maybe missed one

 

[boomdriver]

thank you all for your thoughts.
i will get FSM and check every ground
i know the instrument cluster needs to be rebuilt some times it goes blank and the directinal and high beam lights a guy that rebuilds them said
that was one of the symtems that is y i asked that ?.

has any one ever have a ploblem with with remand mass air parts?
once again thanks

mechanics said they checked all grounds but they did not have FSM maybe missed one

or 3 or 5....if they were thinking battery grounds...they missed most all of them. System or control grounds are at each control servo or control device. They act independantly of each other and carry different signals, so each matters but only to its system that it serves.

There is a collection on the block near the oil filter. More critical than the battery ground cable... Others located near whatever system they service.
I'm glad to see that you made some headway. Progress is good. It motivates us to keep trying.

What did you learn from the repair that got rid of the MAF code?

the wiring is AFU and likely the cause of most of your problems...

These cars came with faulty wire in the harness. GM had some explaining to do to back in the 80s when it was learned that the wire was not going to hold up to the minimum life expectancy thats mandated by the DOT for American made cars. Something like 20 yrs. The plastic insulation cracks and breaks, and gets damaged by various chemicals. I had one side of my engine harness fall apart wigth bare wire everywhere. The other side is still perfect and looks like new while the harness inside the cabin/dash is looking pretty scarey...dried out and dull. Like it will break the next time its touched....

As the other guys have said, it needs to be said again. I cannot stress the importance of having good grounds. Forget the hot wires and positive connections. They are next to meaningless. Hot wires only supply power. They control NOTHING. 100% of the engine controls and sensing is thru grounds because there is less danager of shorting and less damage if there were a short in the system. If they used power to manage all the controls the wire would have to be 4 times bigger, thicker and heavier fuses.Even more relays in the system to complicate things... Another 20 lbs of wire AND the never ending worry of wire fires from all the voltage running everywhere thru the car. Simple hot wires can carry the minimal volts needed without a bunch of resistance issues and the control is done thru grounds....its so simple but so hard for most to understand.
2 types of grounds on a Corvette because its a plastic car with less metal to use as "earth".....there is earth ground, and there is control ground. The later being most common.

Glad you;re making progress. When you get your FSM you can then start to fine tune and make the proper adjustments. I still cannot believe that the shops have been replacing parts without a plan...THEY should have found the burnt wire.......and saved you quite a few hundred dollars...if not thousands.

Hang in there...you;re half way done and it sounds like you;ve made more real progress in the last week than the previous month.

 

[money pit]

what did i learn LOL!!!!:L that i blue $700.00 for nothing. will check grounds this weekend let u know what happens

 

[money pit]

does any one know how and where to get diagnostic computer my enging light comes on goes out comes goes out
ect. every time i go to drive to some one to read code light goes out keep in mind its a 85