Can a burn off module cause a uneven idle? This is a 1985 c4 with tpi and Bosch set up. I had talked with different vette owners and tried a good MAF.....this did not change idle......tps voltage is good with new IAC installed. I have yet to see the car throw a code. It seems like I am getting deeper and deeper into this idle problem and I may throw in the towel and take it a local Corvette specialist.

cmj

 

Have you checked the wires?

 

'85 ?

and you have not changed the injectors?



$125 for the diagnosis and another $50 for not hanging a smiley face on it..!

I'll PM the address where you can send my check. Jon gets 10% so don't mess around.

 

Burn off module should not have anything to do with idle..The burn off module only comes on for a few seconds to heat up the wire in the maf sensor to clean it off...WW

 

You guys have my head spinning! First and foremost this car was given to me and I have no starting point! 55 k miles! Supposedly the car was given a tune-up 11 years ago with new wires, dist, plugs, etc. I find it hard to believe but not impossible that the wires have failed or even that the dist cap, rotor and coil have failed. Now the injectors may be a whole different story. I can feel them pulse very well. How can you tell if they are bad? This car idles between 670rpm and 540rpm. Just doesn't feel right! On the road it seems to accelerate fine. No codes show, no backfiring......I hate to change parts for the sake of guessing. How do your vettes of 85 with tpi run? How can I check for a failed injector? I have no prior experience with vettes and how they run and feel. Sometimes I think this may be a normal idle for a TPI system. Please hit me with any ideas you can think of......I not afraid to step out of the box.....I just need a good place to start!

cmj

 

Im better at doing this then I am at explaning how to do it... But here goes....First you need a multi meter, this is used to check the injector coil resistence, set the meter to read ohms. You have to remove the cap on top of the injector by pinching the metel clip against the cap and pulling upward..The idea is to put one of the connectors on each of the pins to check the resistance of the coil in the injector..It doesn't matter which way you put the connectors onto the pins, (+ or -) doesn't matter.You should get a reading of around 16 ohms, anything below 12 and the injector is bad..All the injectors should be checked cold and also hot when the engine is at running temp. Hope I didn't forget anything.....Let use know what you get.....WW

Here is a video I just found showing how to check injectors....

http://www.youtube.com/watch?feature...&v=_g8PdrT0MCU


PS...One thing I will tell you is this...When I first got my 89, I had a real bad miss and it turned out to be 2 bad injectors. The injectors in our cars weren't made to handle Ethenol fuels and it eats the epoxy surounding the coils away, this causes them to short out....Chances are pretty good that your injectors aren't in the best of shape...After all,,your car is 27 years old ....

 

The most common cause of poor idle quality is a vacuum leak or EGR valve stuck open.

 

I thank you guys for all the input! I have checked the injectors resistance and they are within specs. I believe I may be over thinking the problem. My son and I have suspected from the beginning that we have a vac leak. We have checked and rechecked for a leak. We will be checking the EGR this afternoon. No codes have ever appeared. If I unplug the MAf codes appear immediately. So engine monitoring seems ok. The engine is running 670-540 rpm at idle in OD. Never quite stalls though. Acceleration is very linear and fast. I do have a friend who is quite gifted at vacuum leaks. He heard and found 3 within a minute on my f150....one up against the firewall which could not even be seen. "To every problem there is a solution." It just takes time!

cmj

 

Geez........I would never have guessed the burn off module was for burning off the platinum in the MAF> Good Call!

cmj

 

I have to agree on the vacuum leak, mainly because after arguing for two years that my car did not have one I found a line with a huge hole in it. If I read correctly you said the car was tuned up 11 years ago and has sat ever since? Your plug wires are most likely so dried out the spark is getting out anywhere it wants to.
Also, I sent you a pm about the distributor cover you were looking for, I also have an '85 fsm for sale, if you don't have one and plan on working on your car you will definitely need one. I'll make you a deal on both items if you are interested.
Like WW7 said, you need a multimeter to do anything on these cars, a multimeter and an fsm are the two main tools.

 

I am quite and expert on a vom. I always used a top of the line belkin it was my eyes to troubleshooting. I go nowhere with out it. The problem is that these cars are using multiple sensors to adjust engine if not running correctly......the Ecm may or not takeover to give you a feel that it is running correctly. (LOOP)

 

Hooking up a scanner can tell you if alot of the sensors your car uses are working properly..I have both a AutoXray 6000 scanner, and Datamaster on my laptop..Both will show you if the O2 sensor, Maf sensor, Throttle position sensor, IAC, Coolant temp sensor and a few other important thing are working properly. It also shows if your Air/Fuel mixture is right and shows the timing and timing advance..Maybe you should hook up a scanner to check everything out...WW

 

Hi.
I have an 85 and had idle problems.
I don't think that the burnoff module can be the reason for the bad idle especially that you tried with a goodMAF.
The reason for bad idle is often vacuum leak. With an idle that varies and even make the engine stall can often be the EGR valve sitting under the plenum. The valve pintle can be stuck due to desposits or if the car has been stored due to corrosion. Another fault in the EGR is that the vacuum mambrane cannot hold the vacuum long enough.
An easy way to control this is to apply vacuum to the valve and see that it hold the vacuum slowly decreasing but as long as about 20s.
If the injectors are faulty you caan measure the resistance in them. Think it shall be within 14-16ohms. Has to be checked with specs because I do not remember exactly.
If the injectors are leaky you will have turn the engine more by hot start before it starts.

If you have the possibility check if you have vacuum leaks by using either propane or sart gas and spray it over the intake manifol/heads and plenum. If the rpms increase you have a leak there. Do it also between the MAF and throttlebody as the can be a rip in the " corrugated hose".
Good luck.

 

After extensive questioning and research. I finally broke down and removed the plenum for this 85 C4 a not as bad as I thought it was going to be job. The winning answer I believe for my idle problem was a disconnected EGR!.....no codes ever showed, and I don't understand why not? The hose was laying right next to the connector pipe on the EGR. I am not 100% certain I didn't pull it loose but I don't think I did. Tomorrow a new alternator and Egr will arrive and I will put everything back together! It was suggested to me on 4 different occassions that I had a bad EGR or vacuum leak. It certainly looked far more complicated than it was to remove the plenum. Hopefully this will solve the problem. Thanks to all who gave me opionions !

 

been catching up my emails. thank you for the reply. what is and fsm? i do believe i found the idle problem. i took the plenum off and found the egr disconnected. i am 99% sure that i did not pull it loose. new parts will be arriving 8/1 and hopefully this will solve this mystery!

Chris Jordan

 

Factory Service Manual ( the 2 book set from GM) not a $19.95 haynes or childrens manual from auto-groan or pep-boys.

 

Good work, Chris. And thanks for keeping us updated. Replacing the EGR on a 27 year old car is a good idea.

Let us know if this solves the problem.

 

You can sometimes pic up a Factory Service Manual on ebay, I got the one for my 89 for $35.00 WW

 

Im hoping its the injectors, im about to order a set. Mine idles fine but stutters and misses on accleration and floods itself after a drive. But im getting the code 33 for the MAF. Darn vette

 

Hi Cadjaw

A disconnected vacuum to the egr will give codes after ten or twenty minutes on the highway in closed loop operation. (the ecm will look for the temp sensor on the egr )

I had my egr disconnected for many years and only recently enabled it so i didn't get that code all the time.

The vette should start rev to 1,200 rpm and slow to 600rpm when hot it may idle as low as 570 rpm but be fairly stable.
A base idle setting as per service manual is the best way to go, if someone has fiddled with the idle stop screw it wont idle right.
Check the throttle position sensor 0.54 volts d/c at idle and
4.5 volts d/c wide open throttle (middle and top wire)

disconnect the battery and reconnect it to remove any stored codes etc. Remember the ecm will change things to compensate if it sees something is wrong (its the first thing i do when my 85 runs a bit rough and 9 out of 10 times it solves the issue)

Injectors or valve lash too tight can cause rough idle missing and lack of power etc