GMC 1991 s15 Jimmy tuning problem

check the coolant temp sensor and its wiring.

There is a connector.

Really? This is a 1991 model, which was released in 1990. Where's the ODB connector?

Thanks!

Thank you! I replaced the coolant temperature sensor a year ago, but that doesn't mean it hasn't failed again or the wiring isn't messed up.

Replaced the coolant temperature sensor and checked the wiring. No improvement. Any suggestions?

Up under the dash on the drivers side.

I'll have to check my books, but I believe this is the female end:

Male, (on vehicle), end:

On Tue, 30 Mar 2010 11:24:37 -0700 (PDT), Jonathan Ward wrote:

Google for ODBI for your Jimmy. I have a '90 Corsica and it will turn on the check engine light and store codes. Only happened once about 10 years ago, but shorting 2 terminals in a box under the dash flashed the checked engine light to tell me it was a bad O2 sensor. Used a paper clip because I didn't have a reader then. But I think there is a connector for a reader. Is the check engine bulb good? If it is you might not have a code to look at. Hard cranking could be bad starter. Long cranking could be fuel pressure. You have to check pressure to eliminate that. I had that on a 2.8, but the symptoms were the reverse of yours. It would start fine when the pump was cold. You said you had a new pump, but it wouldn't be the first bad one. The high idle might be unrelated or maybe not. A bad fuel pressure regulator cold cause that, another reason to put a pressure gage on it. I had a bad ECU in the 2.8 that would sometimes cause long cranking, and sometimes just kill the engine like you turned the key. Really it just wasn't sending the right signals when temperature was just right, or maybe a bad trace was susceptible to vibration. Couldn't be trouble shot because it was so intermittent, and never stored a code.. Finally just failed completely. Work on one thing at a time. Start with fuel pressure when starting. Don't recall if you mentioned coil packs. Might be worth considering, though bad ones gave me different issues than you have. Somebody mentioned tracking down vacuum leaks - maybe in this thread - that was a good idea. And he had temp related problems too. Keep us posted. This is interesting.

--Vic

No.

The OBD connector is above your right knee as you sit in the car. I'd start by inspecting all the new stuff that was installed... are the connectors clean and tight? Did the throttle body, egr valve, the iac or the tps come loose? Are the throttle shaft bushings sloppy? Voltage drop the ground wires at the tstat housing... below .050 volts? Corroded iat? Make sure the throttle is closing all the way. Throttle cable too tight? Cruise control hanging up? Block off the iac port in the throttle body with the engine running to see if the idle speed drops a bunch. If it does, iac problem, if it doesnt, look for vacuum leaks. Aim a timing light on both injectors (use the coil wire for trigger), is one of them spraying and the other dumping or streaming? I'd also 'tap-test' the ecm while it's misbehaving, lightly whack it with 2 fingers. High tps voltage maybe? Should be .5 iirc Grab a paper clip and let us know what codes come up. Good luck and HTH, Ben

Okay, so here we are a month later with one symptom left and a somewhat different set of conditions:

- No cold starting problem anymore.

- The "service engine soon" light no longer comes on (other than in test mode when the key is turned on).

This is really puzzling because I didn't change anything.

The idle speed is still very high, and that's the only problem I can observe. But it's not consistent. Sometimes it's fine. A couple of things I've observed:

- Unplugging the connector from the idle air control valve has no effect.

- Unplugging the connector from the throttle position sensor will make the idle speed increase...but not always. Manually manipulating the TPS has no effect on idle speed.

In the last year, I've replaced basically everything related to the throttle body injection (including injectors), including related emission control electromechanical parts. I'm kind of down to the point where there's nothing left to change.

Any ideas of what to check would be greatly appreciated. The Chilton's manual on this vehicle is essentially useless.

Many thanks!

-Jonathan Ward

Vacuum leak downstream of the sensors could cause it. Disconnect the vacuum lines from the intake and block the fittings with caps. See if there is any change. Spray around the intake with some ether and listen for a change in engine speed. Toss a vacuum gauge on it and see what you have.

Steve,

Vacuum at the manifold is ~19 inches, very steady. No evidence of any leaks downstream. I couldn't get the reading to change by manipulating vacuum hoses or spraying penetrating oil on potential leak points.

While I was checking it, I disconnected the wires to the throttle position sensor and set them aside. After rising significantly, after about a minute the idle speed dropped to an acceptable level. Plug the TPS back in and the idle roars up again. I replaced the TPS less than two years ago, but I suppose one could certainly go bad. Seems like the lazy man's way is to keep replacing parts until it works, but I'm not sure what else to try.

-Jonathan

What happens if you disconnect both the tps and the iac at the same time?

Just for fun have you disconnected the battery and let it set for an hour or so? I have seen more than one vehicle have a glitch in the ECM which was cured doing this. Also check the ground to the engine and to the battery. A voltage problem could be making the ECM think something is wrong when it isn't.

I would also try to get a scan tool with live data readout and see what the TPS and IAC are actually doing. It could be the IAC isn't closing enough to drop the idle.