LQ1 High Idle

[Padgett]

My '93 LQ1 is idling at 1800 rpm for a few minutes/blocks after an 80F start then it settles down to 800 rpm. Getting an occasional code 35 but SES not staying on. Cleaned the IAC, cleaned the throttle plate, PCV hoses look good, no audible hissing. Am feeling a vacuum on the plastic hose that goes into the throttle body to air cleaner box elbow but blocking has no effect.

 

Any suggestions ? Vacuum leak seems most likely but everything (HVAC, cruise control) that is usually sensitive to a vac leak is working.

Edited April 20, 2016 by Padgett

[Psych0matt]

Vacuum leak, possibly intake gaskets (same issue on mine, turned out to be intake gaskets)

[Padgett]

OK. What is puzzling is the eventual drop to 850 rpm once it warms up. It is as if something is commanding the high idle. Guess I need to check the MAP and IAC readings high and low.

[Galaxie500XL]

You might also want to check the TPS voltages, if you have a scantool, this works well. I had a similar issue with my LQ1 a few years ago, and discovered a bad spot in the throttle position sensor was the issue.

[GOT2B GM]

lower intake gaskets

[Padgett]

OK IAC is sitting on zero but by the time coolant gets to 120F rpm is down to 1200 (still open loop, the TPS is sitting on .37v (good), MAP is 10-12 in/hg (about 5 psi), advance is sitting on 18 degrees.

 

If the IAC is not controlling it, what is ?

 

Does anyone have a TunerCat map of a LQ1 (1441 Prom) I might look at ?

 

ps just ordered the upper and lower gasket set from Rock Auto, should have next Tuesday.

 

pps just a thought but aluminum expands when hot. Cold be a minor manifold leak that seals when the engine warms up ?

Edited April 20, 2016 by Padgett

[digitaloutsider]

Willing to bet it's the gaskets.

[Galaxie500XL]

Could be.  The last lower gasket I had to replace also was resulting in a coolant leak.  Careful pointing of a flashlight down below the plenum showed a small amount of coolant pooling, especially around the distributor plug on the driver's side.

[mfewtrail]

A self-sealing leak is certainly possible once temperatures have changed enough. Brake parts cleaner is good for finding vacuum leaks. Spray any potential areas and listen for the engine to rev up.

[Padgett]

Then again it may just need to relearn after cleaning. Last time I started it cold, revs came down to about 1200 pretty quickly.

[Psych0matt]

Intake gaskets...

[Padgett]

OK, they'll be here next Tuesday.

[Padgett]

Now more confused than ever. Idling in garage everything is fine, start "cold" (75F). Above 2k for few seconds then drops quickly to within 200 rpm of desired. Hot idle within 50 rpm.

 

Then go for a drive. At three blocks took screenshot below. Everything looks good and no codes but is high. Seems to drop momentarily then comes back up. Three blocks later idle is right on.

 

Sure seems like a vacuum leak but only briefly when driving ?

 

Ideas ? (near final display).

 

Edited May 6, 2016 by Padgett

[Galaxie500XL]

Sticking IAC?  I know you've cleaned it, but perhaps it's not closing down as far as it should.

[Padgett]

Tried tapping on the IAC - no change. Get warm/shut down/restart - back to normal. Wondering if a hose is off. Is there a hose that has vaccuum when cold but shuts off when warm ? Heard a "click" and idle dropped. 

 

Everything seems to work. Only code is a 35 that is triggered by an idle that is above 1500 for a while.

 

Was also playing with TunerPro RT which is why the different displays. Haven't finalized yet.

 

Sudden change also does not match an intake leak

 

High Idle

 

Close to nermal after stutdown/restart

 

 

[Padgett]

Oh this is crazy, start cold goes to 2500 then drops to 1250, start driving and all over the place for a mile, sometimes over 2000, other around 1500. Seems random Then suddenly the idle is fine at 875-900 and stays that way. Nothing consistent.

 

Easiest thing to do is to replace the IAC. Seems to be available from a number of suppliers from $15 to $80 (AC Delco). Any recommendations or does it matter ?

 

ps p/n  17112349 (OEM), 217418, 21812, AC170

Edited May 11, 2016 by Padgett

[wlkstout]

Any luck with new IAC?

[Padgett]

Not initially but may need to learn. Also reprogrammed the PROM but did not touch the cold idle. Seemed to work correctly last night and will see later this morning.

[Nas Escobar]

Let me know what happens to you with this issue. I have the same thing going on with my 93 Z34.

 

Sometimes it will start normal, drive it on the interstate for a while (bout 40 minutes) get back into the city and it won't idle right. If I stop it will take a minute or 2 in order to drop down to the 800 range. Other times it will start high revved and stay there. Idle drops by 1000 when in gear compared to when in neutral and park. Sometimes the engine bounces off the rev limiter in park, idles at 2000rpm in gear.

[Padgett]

Well took it to Bible Study and a few errands and ran perfectly. Temp quickly stabilized at 183-186F, one fan in at 185 on/180 off) AC Off and both AC On. Have not had a chance to run up to "Highway Mode" as yet.

 

Interesting thing was that idle was stable and smooth and within 100 rpm of command.

 

When I had the seating issue of the PROM in the ECM yesterday I noticed a very high idle, the same as a disconnected IAC & in the 2500-3000 rpm range, wonder if a bad PROM contact was putting it in "limp home" mode. Once I verified the PROM, rang out and verified the connector, reseated everything, and started idle was very smooth both cold (1250 rpm) and warmed up (875 rpm).

 

Set for both on above 100F when AC is on to improve cabin cooling.

[Nas Escobar]

Interesting... I've suspected the ECM for a while but never knew how to confirm it.

 

Funny thing is I have a computer out of a 92 Z34. Might swap em and see what happens.

[Padgett]

It is to laugh. Got everything instrumented (was just using the OTC2000 yesterday) and it ran near perfectly. Cold idle a little high but once in closed loop was staying +/- 100 rpm of command. Ran just as nice as could be. Warm out so AC on.

 

Cold idle

 

Coasting about 10 mph in neutral

 

Stayed under 186F even when meandering in a parking lot. Reprogrammed for both fans to be on with AC. Never saw DFCO come on.

 

High cold idle with low IAC and low MAP makes me wonder about a vacuum leak but must not be big.

 

Still feels a little down on power compared to the Crossie but is a later generation 6.

Edited May 26, 2016 by Padgett

[Padgett]

Good news and bad news:

 

Bad news, the high cold idle is back. Sitting on 1750 with 888 rpm command, no IAC, and after a minute.

 

 

Good news: Highway mode is working properly and engine is staying in the 180's even on a 91F day (will have lots more in the 90s as we go through the 200 days of summer. Also once warm idle is fine. Have the intake gaskets just have a 3800 that needs freeze plugs in line first since more annoying than anything.

 

 

Now if I can just figure out how to get the %$#*( logging working in TunerPro RT so I don't keep having to find alt-prtscr while driving then having to pull over to save it (one at a time)...

 

Wonder why the factory put Highway Mode into the programming then turned it off. Felt just fine on road, may lean out a bit more, usually when you can keep cool you can run leaner.

[mfewtrail]

Emissions is the reason for highway mode/lean cruise not being activated from the factory.

[Padgett]

Pontiac got in trouble once before (73) about having an emissions system with a timer that turned off after the test was done. Highway emissions have always been much lower than cold start up and slow bad traffic. LQ1 with a 3.67 axle is never going to have great MPG and needs all the help it can get. 

 

Know of any cars that have it turned on from the factory ? Back in the day I remember 3.1 Centuries had great MPG and not bad power.