Engine Idle Surge
 

My '89GT's idle surges up to 2500 RPM and stays there until I turn the ignition switch on and off. I've learned to do this quickly so as not to stall the motor. I've replaced and correctly set my TPS. What's interesting to me is that the problem gets worse in the rain or when it is very humid out. Recently the problem ONLY surfaces in the rain. Any suggestions??

I'd start with the free stuff like cleaning the throttle body and EGR valve with carb cleaner. You have to remove these parts to get all the gunk out but it's a really easy job to do as long as you remember where everything goes.

If you don't have a K&N air filter get one.

Then change your cap and rotor and do a regular tune-up. I've known a distributer cap to cause idle problems in the rain.

I also had a high idle problem just recently due to EGR dust build-up inside the throttle body causing the throttle plate to stick open a bit, but had nothing to do with rain.

Did you reset your computer after installing the new TPS? Disconnect the battery for at least a half hour to reset it if you haven't.

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90 Mustang LX 5.0
My Ride*My Site

Get a code scanner and post results, or clean the IAB on the side of the TB.

I'll give cleaning out the throttle body a try. I did reset the computer after I changed the TPS. I bought a code scanner, I just have to figure out how to use it!!

I posted the same question not too long ago. The solution for me was one of the following (not sure which because I did all three at once)

Set TPS to correct voltage.

Check for vacuum leaks (I had one on the passenger side behind the strut tower)

Disconnect and clean the 10-pin connector on the back of the motor

One of the three worked for me for the exact same problem...good luck

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1992 5.0 LX, Edelbrock 6037 Heads, Cobra Intake,E-303, 1.6 Roller Rockers,65MM Holley TB, 75 MM Pro M Mass Air, 190 LPH FP w/ 24 lb injectors,BBK equal length shorties,offroad h-pipe,Mac Flowpath Mufflers, Pro 5.0, 3.73's,Motorsport "C" springs,welded subframe connectors, Ponies w/ 225/50 Nittos, 93 Cobra Spoiler

Daily driver: 89 Coupe with 205K plus, BBK shorties,offroad h-pipe,2-chamber flowmasters with dumps, cut and welded stock shifter...and it runs like a champ.

Thanks Green92LX. Just a few questions. Where exactly is the 10-pin connector and what did you use to clean it? Also, what is the best way to check for vacuum leaks?

The 10-pin connector is the wiring harness that connects to your motor right next to the EGR, in that space behind the intake. It should be white and black. I used brake kleen to clean mine, its supposed to be safe for electronics, but they do sell cleaner for this application. Take it apart and check to see if there is any corrosion on the connections themselves, not just the outside of the connectors.
Vacuum leaks are a little harder. I was lucky because when I started to play around in there when I found my leak,one of mine broke off with minimal effort. I changed that line and was done with it. If you do have a leak, you should be able to hear it if you pay VERY close attention. Just be careful with those lines. The hardened plastic lines break easy, even if they are OK before you start jiggling them around.

Let me know if you have any other questions...


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1992 5.0 LX, Edelbrock 6037 Heads, Cobra Intake,E-303, 1.6 Roller Rockers,65MM Holley TB, 75 MM Pro M Mass Air, 190 LPH FP w/ 24 lb injectors,BBK equal length shorties,offroad h-pipe,Mac Flowpath Mufflers, Pro 5.0, 3.73's,Motorsport "C" springs,welded subframe connectors, Ponies w/ 225/50 Nittos, 93 Cobra Spoiler

Daily driver: 89 Coupe with 205K plus, BBK shorties,offroad h-pipe,2-chamber flowmasters with dumps, cut and welded stock shifter...and it runs like a champ.

RMARIANI - Have you tried this? While it is idling high, disconnect the Idle Air Bypass Valve. If the engine idles down, you know it is the computer. I've never heard of a vacuum problem making the computer increase the idle speed. And there is no reason for it to either. If the computer is causing the motor to idle high, it is either incorrectly set TPS. Or a ground problem somewhere between your TPS and your computer. Best place for that problem to occur is the connector that GreenLX was talking about. I have cleaned my connector and have no more problems at all. I am almost positive that this is the problem.

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Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!

I would just like to thank everyone for their suggestions. I'll give them a try this afternoon when I get out of work. I'll let you know tomorrow how it turns out.

Just to defend myself here, I only listed the three possible causes for the mere fact that I did all three at once. I do believe that it is the connector as well, but I had to be honest with my approach when I solved the problem on my own car.

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1992 5.0 LX, Edelbrock 6037 Heads, Cobra Intake,E-303, 1.6 Roller Rockers,65MM Holley TB, 75 MM Pro M Mass Air, 190 LPH FP w/ 24 lb injectors,BBK equal length shorties,offroad h-pipe,Mac Flowpath Mufflers, Pro 5.0, 3.73's,Motorsport "C" springs,welded subframe connectors, Ponies w/ 225/50 Nittos, 93 Cobra Spoiler

Daily driver: 89 Coupe with 205K plus, BBK shorties,offroad h-pipe,2-chamber flowmasters with dumps, cut and welded stock shifter...and it runs like a champ.

Well I cleaned the 10-pin connector(s), there were actually 3 of them. Two black and one white. The two behind the manifold were a little dirty (greenish color). The other one had a brownish paste over the pins. I thought it was dielectric grease, but I wondered why the other two didn't have the grease. I cleaned all three and my car is running great. I took it out last night for about a 1/2 hour and this morning my ride to work (about 45 minutes) and no surge. The car actually seems to be running better all around. Do you guys think I should apply dielectric grease to the pins. Its used to protect contacts from moisture and corrosion. Thanks again for the help!!

You might want to, if would help keep it from getting corroded, but I don't know about messing with something that is working correctly. I'm still considering running three seperate wires for my TPS, one straight to the ground pin on the computer, one to the signal on the computer, and one to the +V from the computer. I would think that would fix it for sure. Anyway, glad to hear it's working better, That's kinda weird about having three connectors, I have two, everyone I've seen has two, except a friend of mine has four. It's like someone has spliced half of a wire harness in or something, I donno. Strange.

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Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!

Congrats...glad we could help.

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1992 5.0 LX, Edelbrock 6037 Heads, Cobra Intake,E-303, 1.6 Roller Rockers,65MM Holley TB, 75 MM Pro M Mass Air, 190 LPH FP w/ 24 lb injectors,BBK equal length shorties,offroad h-pipe,Mac Flowpath Mufflers, Pro 5.0, 3.73's,Motorsport "C" springs,welded subframe connectors, Ponies w/ 225/50 Nittos, 93 Cobra Spoiler

Daily driver: 89 Coupe with 205K plus, BBK shorties,offroad h-pipe,2-chamber flowmasters with dumps, cut and welded stock shifter...and it runs like a champ.