Hi guys,

My 1995 Camaro LT1/T56 combo transplant in a 1984 Jaguar xj6 is suffering from high idle. It idles at around 1400-1500. I have done the following:

I investigated for an intake leak, and didn't find one -- I replaced the intake gaskets again for good measure and torqued everything down to spec. Still it is idling high.

I replaced the IAC.

I've checked for other intake leaks at the brake booster.

I replaced my MAF.

I tried Re-setting the IAC (holding the accelerator pedal down slightly on startup and then letting go after 5-10 seconds)


I'm not sure what else it could be -- the car threw one code (the fans clicked on, so I decided to check out what the code was) but when I scanned the PCM, there were no trouble codes stored.

Since my Speedo isn't working (i tried solving that one a couple weeks ago) i could have a VSS problem -- does the VSS partially control idle?

I don't know what the problem could be -- does anyone have any suggestions? I feel like i've exhausted all options. One of my friends says it could be that the timing is off, but the engine isn't running rough, just a high idle.

What else could I try? Since the car is a conversion most mechanics are reluctant to work on it, and I've done all the other work on the darn thing I'd like to figure it out on my own, but it's diving me nuts!

Thanks for any help!

 

Have you verified the throttle blades are not sticking open, or that the throttle stop screw is not out too far? Have you used a scanner to observe the IAC counts, and if so how are they responding to the hight idle?

Is it possible your coolant temp sensor (the one in the water pump housing) is showing an incorrect low coolant temp? The PCM elevates the idle RPM for low coolant temp, because it assumes its in "cold start" mode. Aso richens the mixture, which would be another problem.

The PCM also elevates the idle RPM based on vehicle speed. It holds the RPM above the normal idle speed as long as the vehicle is moving (as read from the VSS sensor), so that might be related.

 

The throttle body was fine when I had it on the other engine (1996 caprice). I adjusted the throttle stop screw, and it still keeps at a solid 1500rpm. It's really weird -- no loping, and no floating high RPMs when i let off the gas pedal when I rev (supposedly this could mean a vacuum leak).

The sensor in the waterpump is giving good readings, at least according to FreeScan -- i can check the IAC counts next time I get a reading to see exactly what it's putting out.

I suppose the PCM could be thinking that it is going at a higher speed because of a VSS malfunction -- i'm still not sure how to address that issue (I'm using a 93 T56 with a 95 Camaro tune... i've read different forum posts on what type of signal the 93 T56 trannies outputted vs the later ones... i adjusted the PPR to the 93's reluctor ring tooth count in Tuner Cats... still no dice).

I also checked for any exhaust leaks, and there doesn't appear to be any. Any other things I can check for?

 

Did you check the throttle cable?

Is the TPS voltage below 1.0V?

 

I have the same problem in my 95 z28. Although my temp sensor isnt working, would that be my problem? Has to be replaced anyway, but just wondering. Sorry to threadjack.

 

Throttle cable looks fine -- i'm going to check the TPS voltage and see what it's at. Do i Need to have the car running to check the voltage, or can I just have the car set to "on"? I just don't want to run the car very much while it's high idling... worried about too much gas in there.

 

I see a problem and have a suggestion.

First, if you had a DTC and fans came on but you couldn't find the code, that is an issue you have to resolve. If my scanner could not find a code I thought was there, I would force a code and be sure I could see it.

The suggestion is basically what Fred said by the way of IAC counts. If you can't find the counts, try pluggin the IAC port and see what it does. If you still have a high idle, you know it must be from something else. Like an air leak or TB issue etc.....

If it does go down or even shut off, you know to focus on the PCM getting information that is incorrect or you have a IAC issue. Maybe wiring, maybe sticking etc......

So basically see if it's a mechanical issue or command issue by eliminating one at a time.

 

I scanned the PCM again this morning and "Error Code 34 -- MAP Low" popped up in the trouble code window in FreeScan. I researched the trouble code, and it is set when:

DTC - 34 MAP sensor circuit error (signal low indicating high vacuum)

Code 33- MAP sensor signal voltage high. Check vacuum hoses to MAP sensor. Check wiring and connections for any shorts or opens. Replace sensor.

(i pasted the second error code because the flowchart said to check 33 if 34 comes up)

There isn't much information on testing the MAP sensor that I can find -- would it be easier to just replace the sensor? Or are there other avenues to pursue to resolve the issue?

 

Check the MAP harness connector. They seem to dry out and crumble producing erratic readings due to intermittent contact.

You can check the TPS voltage with the key in the "run" position, without starting the engine.

 

would the use of a vaccum gauge confirm the map readings in order to confirm a V leak?

 

I haven't been able to check the car just yet (getting off work too late and it's getting dark out earlier these days) but would DTC 34 be a signifier of a high idle? I can only assume that the little seal around the MAP (orange rubber seal) is messed up enough to let some air seep through... I haven't found any info on whether this trouble code could also be tied to a high idle.

I suppose if the MAP is defective, that would mess with the PCM readings... it's just that the idle is so steady (just high)!

I'll check the TPS voltage today at lunch.

And yes, I do have a MityVac so I could do some diagnostics with that -- what are the Hg readings that I need to look for for proper MAP functionality?

 

Soooo here's the results:

By the time the car warmed up to it's desired idle speed of 850, I had an IAC count from 50 to zero. My MAP was at .288 and the TPS was at .28v. One interesting thing that I noticed was my O2 sensors were giving slightly different readings -- Left Bank was at .92v whereas the Right Bank was .7v -- not too much of a difference but I was curious as to why this was happening.

When all is said and done, I was still idling at 1300 when the car was warmed up to this point. I swapped the MAP out with another unit -- I didn't throw the code again (with either unit) but the new seal may have had something to do with that.

One other interesting thing -- at my brake booster, i noticed a slight hiss after i turned the car off. I thought perhaps i had solved the problem, but when i capped the end of the brake booster hose there was still no change (nothing that i could read off the tach anyways).

Any ideas anyone?

 

I got some results from me scanning the PCM at lunch just now. I used TTS DataMaster.

A) TPS voltage is at .9v -- I must have been mistaken, or had a funky reading before when I used FreeScan. I used FreeScan again to double check DataMaster's data, and it came up with .9v at 0%TPS. When i stepped on the accelerator, the voltage went up a bit, to over 1.04, but I didn't hold it long enough to see exactly what it went to. When I revved the engine, it hit around 2800rpm -- very high for not much of a press on the gas pedal.

B) A VSS code popped up -- Code 94, which is no VSS output 4000ppm. I had tuned my VSS in TunerCats to read 11ppr (because I'm using a 1993 T56).

C) I tried plugging the brake booster outlet, the EGR, HVAC -- no change in idle speed.

D) IAC counts still goes down to zero.

I'll keep hunting for vacuum leaks but I don't know where else they could be coming from...

 

Are you getting the rpm reading from the scanner or the Jag's tach? I just ask because an uncorrected 6 cylinder tach would read about 1200rpm on an 8 cylinder engine running at 850 rpm.

What does the scanner think the vehicle speed is?

It takes a pretty good-sized vacuum leak to idle that high....check every vacuum line, including the PCV line, for cracks, etc...

 

I corrected the Jag's tach with the resistor mod -- it reads the same as my PCM reading, within 50rpm or so. Shbox recommended tweaking the TPS to completely rule it out as a problem. I'm also going to check on the throttle body set screw -- i haven't touched it yet, maybe the blades are cracked open slightly. I can't seem to find the vacuum leak still...