Inconsistant Idle
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Joined: Feb 2009
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From: charlottesville Virginia
Inconsistant Idle
I'm not sure how long this problem has been going on, as its never idled perfectly for an extended period of time. Recently, I went to back out of parking place and when the car would go into reverse the idle would be so low the car would stall, same when in drive. I was able to drive the car around by keeping the throttle at around 10k when stopped and the car was fine when driving. On other cars, I've had this issue and it turned out to be the IAC. Replaced the IAC valve and cleaned the port in the throttle body, then went through and set the idle speed and tps. The car still has an idling problem, it searches between 5k and 10k, even when up to temperature. It will act like its fine for a few minutes then go back to searching again. The only code I pull on a regular basis is EGR (32), but occasionally get 31/33 and once 42. The MAF and both relays have been replaced, along with the tps sensor. The previous owner relocated the MAT to the air box, I've read that doesn't do a whole lot for the 86 though. The problem seemed to become worse after installing a reverse flow, cold air intake (through the radiator), although that could just be coincidence. Any ideas of where to start with out buying a bunch of parts?
Burning Brakes
Joined: Nov 2011
Posts: 841
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start by seeing if the IAC is 1 1/8" from gasget to tip. Search this forum for a tech tip tagged "how to adjust your early c4 TPS and idle speed". I downloaded it and printed the 3 pages and once I ran through the procedure my car ran like new. Youll need a voltmeter and a paperclip. If you cant find it, let me know and I'll e-mail it to you.
Burning Brakes
Joined: Nov 2011
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http://www.corvetteforum.com/techtip...?SubTopicID=55 Here's the link,enter the search for "how to adjust your early c4 tps and idle speed". scroll down a little and you'll see it. Follow these instructions and let us know how it goes.
Last edited by caddyboy84; Mar 26, 2012 at 10:37 AM.
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From: charlottesville Virginia
Caddyboy-link doesnt work. I do know which link you are referring to and have copies of it all over the garage and downloaded on my phone. I have followed the instructions and repeated them multiple times this weekend, nothing seemed to improve.
Burning Brakes
Joined: Nov 2011
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Does the IAC work? Are all the injecters fire properly? How about the timing, I keep mine at 8 degrees. Believe it or not, try cleaning the MAF with a good spray cleaner as well. The air cleaner can effect idle also.
Last edited by caddyboy84; Mar 26, 2012 at 10:45 AM.
Burning Brakes
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Thread Starter
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Joined: Feb 2009
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From: charlottesville Virginia
I do have a K&N, not sure when it was cleaned last so I guess that will be a good first step. Not sure how de-screening the MAF would effect idle, but I do have an extra one laying around, so it may be worth a shot. How would you check if the IAC works, I assume it does since the car does not idle with it unplugged and does so, but inconstantly, with it plugged in. I guess timing and fuel injectors are next to check out.
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Burning Brakes
Joined: Nov 2011
Posts: 841
Likes: 1
I do have a K&N, not sure when it was cleaned last so I guess that will be a good first step. Not sure how de-screening the MAF would effect idle, but I do have an extra one laying around, so it may be worth a shot. How would you check if the IAC works, I assume it does since the car does not idle with it unplugged and does so, but inconstantly, with it plugged in. I guess timing and fuel injectors are next to check out.
Last edited by caddyboy84; Mar 26, 2012 at 11:39 AM.
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Joined: Feb 2009
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From: charlottesville Virginia
How would this filter be "starving the car for air"? Never heard of that before, only that they let too much dirt through sometimes, and if thats the case do I go with the less flowing paper filter to fix the problem? That argument doesn't really hold up anyways because the problem is at idle, if it was a lack of air problem there would be a problem at full throttle (more fuel needs more air). If it were the plugs and wires, wouldn't the problem be consistant and not solely at idle? If it was the oil from the filter messing with the MAF wouldn't I receive a code? And an OBD II scanner, even with attachments is only going to give me the same info as doing the paperclip reading.
Race Director
Joined: Apr 2002
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From: Anaheim CA
On my EPROM (BUA) error code 31 is not used, so I don't know about that one.
Error Code 33 is MAF sensor high. That can be caused if you disconnected the MAF sensor while troubleshooting.
Error Code 42 is EST error. That is usually caused by disconnecting the EST connector while setting the timing.
You need to clear the error codes and then check to see if any reappear. Clear error codes by disconnecting this connector (#2 in the pic) for a second or so:
Race Director
Joined: Apr 2002
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From: Anaheim CA
I got sidetracked by the error codes...
Some causes of random idle are:
Bad CTS (Coolant Temperature Sensor), located on the front of the intake manifold under the throttle body.
Bad O2 sensor.
Bad TPS.
Vacuum leak.
"Bad" in this case means the sensor itself could be defective or the wiring could have problems (like not being connected, broken or shorted to ground).
You really need a scan by computer-based software to find out what the ECM is seeing.
Some causes of random idle are:
Bad CTS (Coolant Temperature Sensor), located on the front of the intake manifold under the throttle body.
Bad O2 sensor.
Bad TPS.
Vacuum leak.
"Bad" in this case means the sensor itself could be defective or the wiring could have problems (like not being connected, broken or shorted to ground).
You really need a scan by computer-based software to find out what the ECM is seeing.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
I think I put out bad information again (I hate when that happens). I said you could time your car at 5000 RPM, but I got reminded that the ignition module has a timing curve built in to it, so that's not true.
I want to experiment with this but I can't run my car right now to test it. It should be fixed tomorrow, so we'll see.
I think the best person to answer this would be 69427. Since he designed a replacement IC for the ignition module he should know. That was 30 years ago, so he might not remember... I'll send him a PM and see what he says.
I want to experiment with this but I can't run my car right now to test it. It should be fixed tomorrow, so we'll see.
I think the best person to answer this would be 69427. Since he designed a replacement IC for the ignition module he should know. That was 30 years ago, so he might not remember... I'll send him a PM and see what he says.
Last edited by Cliff Harris; Mar 28, 2012 at 03:10 AM. Reason: Reformatted to clarify.
Racer
Joined: May 2011
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The K&N is a good filter but you do have to take care of the maf wire cleaning and you can not over oil the filter, if you do, yo will have problems.
I have had to "fix" many cars because the owners put a K&N filter in and oil was just dripping out of the filter and housing!
A lot of the "old" cars ran a oil bath filter but the paper was introduced 4 ease of maintenance and environmental reasons.
I have had to "fix" many cars because the owners put a K&N filter in and oil was just dripping out of the filter and housing!
A lot of the "old" cars ran a oil bath filter but the paper was introduced 4 ease of maintenance and environmental reasons.
Thread Starter
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Joined: Feb 2009
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From: charlottesville Virginia
UPDATE
Adjusted the idle again and it started running better, still getting code 42. Checked the timing which was set at 2* and moved it to 6* per the manual. The car ran terrible so it was put back to 2* for now. The distributor may be off a cog, so I'm not positive TDC is actually TDC. The car now idles perfect, but when accelerating it does not seem as responsive as it used to, it barely moves when accelerating lightly but still takes off pretty good when getting into it hard. On a cold start it still is very rough and hard to drive from the unresponsiveness, but as it warms up it gets more "normal". Also, my mpgs are way lower than they used to be, highway im getting 15 now and around town 12. Before this issue came up I could get 25 highway and 18 in town. Ideas?
Cliff: What software would you recommend to scan this and what kind of tool is needed for it? I was under the impression that the code number is about all you can get from scanning an OBD1 car.
Adjusted the idle again and it started running better, still getting code 42. Checked the timing which was set at 2* and moved it to 6* per the manual. The car ran terrible so it was put back to 2* for now. The distributor may be off a cog, so I'm not positive TDC is actually TDC. The car now idles perfect, but when accelerating it does not seem as responsive as it used to, it barely moves when accelerating lightly but still takes off pretty good when getting into it hard. On a cold start it still is very rough and hard to drive from the unresponsiveness, but as it warms up it gets more "normal". Also, my mpgs are way lower than they used to be, highway im getting 15 now and around town 12. Before this issue came up I could get 25 highway and 18 in town. Ideas?
Cliff: What software would you recommend to scan this and what kind of tool is needed for it? I was under the impression that the code number is about all you can get from scanning an OBD1 car.
