1987 Vette with severe miss when hot.

[Michael]

1987 Vette with sever miss when hot.

Hey guys, great forum... I'm glad I was referred. I hope that someone out there could help with my current 1987 vette issue. Sorry for the length of explanation, I'm deperate and my mechanic cannot seem to figure the problem out. Please help.

1987 Corvette / Vin. # 8



During the first part of January I was driving on the freeway and noticed that when I stomped on the gas, the car slightly missed before kicking into high gear. This was subtle and intermittent so I didn’t pay too much attention to it and figured it could just be a spark plug or plug wire issue which I was going to be replacing soon anyway.



The problem became less intermittent, and I started noticing the miss when taking off and shifting into lower gears (this is an automatic by the way) so I decided to do a tune up thinking that would be my problem. The following is a list of items replaced during the tune-up.



- Spark plugs

- Plug wires

- Distributor Cap

- Rotor

- O2 Sensor



The timing was set to 6 degrees per spec. and it seemed to run fine right after the tune up. But when the car warmed up, the miss was noticeable still. I took it back to the mechanic and he checked all of the setting with the timing but the computer was reporting that the Throttle Positioning Sensor (TPS) was too low. So that was replaced.



- Throttle Positioning Sensor



This issue still didn’t resolve my problem. After going over the car again, he then thought that what was reporting the error to the ECM that the TPS he suggested that I replace the Coolant temperature that reports to the ECM. That was replaced.



- Coolant temperature sensor



This still did not fix the problem. The car misses slightly when cold, but gradually misses as it became hot to the point where smoke was exiting the exhaust indicating that it was running rich. This error was reported by the 02 sensor as well. The following parts were replaced since then with no change in condition. The car still ran severely rough specially when in gear to the point where it would die and not start until we waited for it to cool down.



- Engine Control Module (ECM) keeping the original PROM chip.

- Replaced the 02 Sensor again to make sure the first was not defective.

- Fuel Filter

- Coil

- Ignition Control Module

- EGR temperature switch (sits on the EGR pipe)



Still this did not fix my problem. The car runs strong when cool, and misses hot where it is could not be driven.



HISTORY



During last summer I replaced the following parts prior to these symptom.



- Mass Air Flow Sensor (MAF)

- All Injectors replaced

- EGR

 

[Daryllawman]

Hello....I don't have the answer for you, but there will be someone along soon that can help. Good people with alot of knowledge in here....I just wanted to welcome you to the Corvette Action Center......Enjoy !!!!!!

 

[hoosier]

intermittent miss

Hi Mike,
Just a shot in the dark ,But you might try tightning up on the spark plug gap .
I had a similar problem with a car of mine ,and found out the gap was too wide and blowing out the fire.

Barry

 

[WhalePirot]

Search the forum (or another source) for the thread about 'reading' the plugs. At least one is likely blackish from a rich condition. That should narrow down the problem to the cylinder(s) that are not firing properly.

Also, check that the 5 and 7 wires don't parallel. Do you see or hear any arcing from the wires when running the car in a dark environment? Did any plugs get cracked?

The sensors can be checked rather simply if a tech will take the time, not throw parts at it. The TPS voltage MUST be adjusted, especially after replacement. It is a simple, but essential procedure.

A start, these, but some shops can pull a LOT more from the ECM than trouble codes. The equipment is rather pricey, but may be in your future if the basic and simple stuff doesn't solve your problem.

Google turned up more than the following, as an intro to reading the plugs----
http://www.strappe.com/plugs.html
http://www.nightrider.com/biketech/spkplghnbook.htm#We
http://www.theultralightplace.com/sparkplugs.htm
http://www.atlanticjetsports.com/_techtalk/00000005.htm

 

[MBDiagMan]

The answer to incorrect TPS voltage is usually not REPLACING the TPS, rather ADJUSTING IT. I would start by checking and adjusting the TPS to the recommended 54 millivolts. I would also check and adjust timing and check fuel pressure. This would all be good to do and would give a good baseline from which to start other troubleshooting if necessary.

Are there any codes set?

Hope this helps,

 

[spyysee]

Check the compression of each cylinder
check the fuel pump pressure
good luck and be sure to let us know!

 

[Moonunit 451]

I'm wondering about the MAF. If you can access another one you can swap it out pretty quick to rule that out. A couple other things, injectors, leakdown test and pulse width I think is the term along with fuel pressure and leakdown tests as mentioned. And one more thing is frequently a problem, not neccesarily as you describe, but plugged cat can cause real problems. Keep us informed and try not to replace any more parts unless you can confirm they are bad first. Good Luck, the folks here WILL get you through this if you're patient and communicate well. Welcome!

 

[spyysee]

the cat...good thinking!

 

[Michael]

Hey guys, thanks for the help. I am fortunate to have found a forum where others out there really do care...

I have changed the Catalytic converter thinking it was clogged causing the car to eventually stall. This was done just last month. Still the problem existed.
I stopped by a Corvette mechanic shop that I haven't had any experience with regards to quality and knowledge of workmanship, and the mechanic there suggested two possibilities.

1) The Ignition control module that I bought should have been an AC Delco brand. The brand I bought was from B&B Auto. He thought that this could be too cheap of a part and doesn't work well with the kind of car I had.

2) The "Timer Core" which he also states is the same as the "Pick-up coil" should be replaced and furthermore I should have my distributor rebuilt.

Anyone out there think this could be my problem? I'd hate to invest in this direction and find myself in the same situation.

 

[Michael]

Well guys, after all that and an idiot for a mechanic... I finally resolved to take my car to the shop. The mechanic that worked on it initially told me that the fuel pressure was within spec... what he failed to do was rev the engine so that when the injectors were taking gas the pressure was still stable, and in fact they were'nt. I WON'T BE GOING BACK TO THAT LOSER AGAIN!

The problem was my fuel pump. It showed good pressure when idling, but under a significant load and for some reason when the car was warm, the pump became weak and the pressure dropped significantly. The car is fixed now, just in time for the weekend.

Thank you all for your help, suggestions and advices. What a great place.

 

[tnovot]

The problem was my fuel pump. It showed good pressure when idling, but under a significant load and for some reason when the car was warm, the pump became weak and the pressure dropped significantly.


20/20 Hindsight is wonderful. It all makes sense NOW.
A liquid is more likely to boil (vaporize) at a lower pressure for any given temp.
OR, it will more likely vaporize at a higher temp given a constant pressure.
In any event, it sounds like low fuel pressure and high temps were combining to give you vapor lock. Higher engine temps probably did not make your pump any weaker than it already was. The low fuel pressure problem just showed up better at higher temps.
Glad you have this problem resolved. Hopefully, your experience will be helpful to someone else.

 

[WhalePirot]

Glad it's fixed.

BTW, the fuel pressure changes when vacuum moves an internal diaphram.