87 corvette bucks during idle

[rayres74]

I have had my 87 corvette for about 6 months now. I have noticed when on cold start up it races up around 9 to 1200 rpm until warms up then drops to about 600 rpm after approximately 30 seconds. Once warm and you start the engine again it races up again and will lower back down after about 10 seconds every time. Didn't think this was normal. Car drives fine when running down the road it just happens to buck when at idle every 10 to 20 seconds. Thought the engine would know that it was warm and would idle correctly. I have known that the idle should be at about 450 rpm in drive but mine it around 500 to 700 depending on it bucking when at a stop. I am not a mechanic but am definitely mechanically and electronically inclined. Could anyone lead me in a good direction? My engine is totally stock. I am getting a check code of 32 by jumping A B pins. First I get "12" 3 times then it goes to "32" 3 times and back again. I was hoping 12 is just saying that it is in diagnostic mode since that was a general code for GM. Then it showing 32 was the real issue. I also have a 91 gta and it also reads 12 first so I thought this was normal. I hope I am right on the 32 could be 23. Damn I will have to do it again in the morning. Any ideas from anyone? Should it be idling this high or should it really be at 450 in drive? What is the easiest way to set the tps and details on how to do it? My head gaskets were just replaced from an over heat because of a fan relay. He said the check engine had to do with 02 reading but it could be showing that cause of all the coolant blowing out through the exhaust when it happened. He said if it didn't stop coming on he would replace the o2 sensor. Well it hasn't stopped but I believe if you want something done right do it yourself. Just not sure my mechanic is all there with this engine and would like to make sure myself.

 

[boomdriver]

I have had my 87 corvette for about 6 months now. I have noticed when on cold start up it races up around 9 to 1200 rpm until warms up then drops to about 600 rpm after approximately 30 seconds. Once warm and you start the engine again it races up again and will lower back down after about 10 seconds every time. Didn't think this was normal. Car drives fine when running down the road it just happens to buck when at idle every 10 to 20 seconds. Thought the engine would know that it was warm and would idle correctly. I have known that the idle should be at about 450 rpm in drive but mine it around 500 to 700 depending on it bucking when at a stop. I am not a mechanic but am definitely mechanically and electronically inclined. Could anyone lead me in a good direction? My engine is totally stock. I am getting a check code of 32 by jumping A B pins. First I get "12" 3 times then it goes to "32" 3 times and back again. I was hoping 12 is just saying that it is in diagnostic mode since that was a general code for GM. Then it showing 32 was the real issue. I also have a 91 gta and it also reads 12 first so I thought this was normal. I hope I am right on the 32 could be 23. Damn I will have to do it again in the morning. Any ideas from anyone? Should it be idling this high or should it really be at 450 in drive? What is the easiest way to set the tps and details on how to do it? My head gaskets were just replaced from an over heat because of a fan relay. He said the check engine had to do with 02 reading but it could be showing that cause of all the coolant blowing out through the exhaust when it happened. He said if it didn't stop coming on he would replace the o2 sensor. Well it hasn't stopped but I believe if you want something done right do it yourself. Just not sure my mechanic is all there with this engine and would like to make sure myself.

Your code 32 is fairly common being an EGR issue. There are several ways this can set....from the solenoid, the EGR temp/time switch or other eng parameters being out. The first thing would be to visually inspect the EGR vac lines to the solenoid and check its elec plug. Next vac test the EGR valve with a mirror to see if its actually moving when vac is applied. They can stick open an cause a surging idle and make it rough as well. Idle should float when cold based on a/c demands or trans placement but not widely. Normal in gear ...600-700 + 50 for a/c on. Should idle 900+ when cold then settle back at around 600 after the min temps is reached...around 158 (on mine). It also has a low stall torque converter so the trans is grabbing just off idle and the TCC when in 3rd gear makes it act like its locked in all the time. When in 4th (OD) this defeats the TCC under some conditions and allows a different "feel" to the trans. This feature helps the trans grab and react instantly as gas pedal is applied. It may want to groan more while sitting at a stop...but its a Corvette. They want to move. You might have the Trans TV cable too tight also. That forces the shifts to be stretched out and makes t he engine spin up too much before it can shift. It has an effect on idle behavior as well.

Your Corvette motor may look like the GTA or even a Camero TPI but its different. The Vette has 2 egr passages to intake, different PCV and different A.I.R. systems. Yours has a different cold start and there are other subtle fuel/intake differences. It's not going to idle at 450....thats way too low for even a stock TPI.
You may need Jon...All C4's eventually need Jon.

Setting the TPS is simple. Get a volt meter, find the correct 2 wires to tap, set to .54V at closed throttle. Probe in the dk blu & grey wires. Grey is 5V reference voltage and blu is the TPS return signal.

 

[vette079]

Now OTOH, if it was a code 23, it is your TPS sensor.
Just check the codes again and verify IF it is 32 or 23.
You can check/adjust the TPS with ign. on engine off.
If it shows the 0.54 volts at idle and steadily rising along when you open the throttle up to 4.5-5.0 volts, it´s OK.
After that, turn on the parking lights, disconnect the neg. batt. cable for a minute, turn off park lights again, reconnect neg. batt. cable again.
Start and drive the car for a half hour, floor it at least once for at least ten seconds.
See if the code(s) come back.
If the 23 doesn´t come back, the TPS is OK, if the 32 comes back, it could be either the EGR solenoid or clogged vac. lines, or vac leak somewhere.

My 50 cents, you can donate through Paypal if you want... otfl

 

[lt1george]

Make sure you have the correct 195 themostat installed. If it is not the 195 degree themostat in there that will cause the idle to "hunt" for correct idle speed. A simple check, do it. Happened to me years ago on my brand new C4. Thought it was runnning too hot so I changed to a 160 themostat. Caused the same kinda problem you're talking about. Most look right away at the IAC, it could be that. But check that themostat. Let us know how you make out.

 

[rayres74]

Ok guys its been a while and I have fixed many things since the first post. I had a head gasket leak blowing coolant out through exhaust and had that fixed. Since then when everything was put back together my mechanic asked if i noticed the transmission slipping. I said no and it was and has been since I got it fixed. Long story short I finally found out the tv cable was out of adjustment and fixed that yesterday. After my mechanic was suggesting a new transmission it was a 5 second adjustment. Should know better than to trust a mechanic who works at Advance Auto Parts during the Day. Anyway I am glad I am figuring all this stuff out myself. The next was a code 54 so I swapped in a new relay, although I believe my old was good but the wires were exposed at the plug and crossing so I separated those and taped them up. Also had that code 32 and seems the replacement of the sensor wire on egr exhaust fixed that. So now I still have the bucking at idle. It seems worse in drive than in neutral but that maybe because I will feel it more in drive. The interesting thing is is when I come to a stop and sitting there in drive, I can count about 20 seconds and it bucks. Then it smooths out and does it again every 20 seconds exactly on time. I had a code 43 before so I did the whole timing and minimum idle tps adjustment. Took care of that. With all codes gone and transmission tight again getting correct line pressure this thing has come to life. But the buck still occurs. After some reads some say to wiggle that maf which I will try, Unplug 02 sensor, although I know I will get a code and see if it smooths out. Any other ideas?

 

[boomdriver]

Your IAC could be sticking or not working. If the idle stays the same or bumps up slightly when the a/c is turned on the IAC is working...
Mine does the same behavior when stone cold,. but does not rev as high as yours. Mine will go to 1000 and sit for 30 secs on cold start then settle to 600 in park. Once driven its a solid as a rock. I'd have to believe that you could benefit from pulling the throttle body off and cleaning the IAC pintle and seat, and the idle air passages. Next I'd look for air leaks that are causing the ECM confusion...that can send the IAC into spasms. A scan does wonders to show the IAC step counts and what its reacting to, or if IT is the problem. If you haven't bought your FSM yet, so so asap. You'll get more info out of those 2 books than all the wanna be Vette mechanics in your town. Everybody wants to work on a Corvette until they do....Everybody claims they CAN until they do, and can;t. The FSM gives you more info then they have and provides you with the GM method of diagnosing and repair for every system on the car. First class investment for every vette owner.