Like the subject says it idle rough. Actually, it does two things at idle. The first is the rough idle. It's seems to miss on a cylinder every now and then. If you step up the idle it's smooth so my thought is injectors - but I really don't know. The other thing is when I come down from a steady 1500-2000 rpm to idle. Like pushing the clutch and off the gas. It will oscillate 500-1500 rpm. If it doesn't stall it will settle down after a minute or so. This one has me thinking it's the IAC. but the PCM doesn't give me any reading on this so I'm not sure.

history - 3800sc from a 97 grand prix. new fuel pump to support the engine. new fuel filter. new air filter (cone type). It runs great off-idle right up to the rev limiter.

Any suggestions on what this/they might be, if they're related, or how I might diagnose this?

thanks

You may be on to something with the fuel injectors. My buddy's Mustang GT killed a cylinder due to leanout. We sent the injectors to FuelInjectorMan and when they came back the performance charts were, to say the least, eye popping! Just prior to it's death you wouldn't have thought anything was wrong, it even had a decent dyno run. I can't count how many times I've gone crazy on idle issues only to discover dirty/drippy injectors and/or excessive carbon buildup on the valves were causing the problem all along.

Can you scan your engine to see what your LTFT is at idle?

LTFT?

I hope your trying to scare me. Putting a hole through a cylinder do to injectors running lean! shold I take the seriously and spend the cash on new ones?

LTFT - Long Term Fuel Trim - a percentage of how much more or less fuel is being commanded with a variance I've seen of -16 to +16. Typically you want this number at 0. If it's high and positive, it's adding fuel because it's seeing a lean condition. If it's negative, it's pulling fuel because it thinks the engine is getting too much or is running rich. Rough running at idle can often times mean you have a vacuum leak. For me my ltft was sitting around +10 to +15 and we finally found out that it was an injector o'ring that was leaking causing a small but noticeable vacuum leak. I also later found out that my l67 doesn't have very good compression numbers which lead to a small misfire that would throw a code (sometimes a blinking SES code, other times a code that wouldn't trip the SES light). I'm still in the process of fixing that last part .

If you had a scanner that can hook to the engine, it should be able to pull up this number as well as a few others that might help you troubleshoot the problem.

BTW, my l67 would exhibit the same behavior as yours when you press in on the clutch and let off on the gas (approaching a stop light). Fixing the vacuum leak lessened the severity and after I barely notice it doing it.

[This message has been edited by JamesCurtis (edited 05-30-2011).]

Excelent info on the LTFT. Yes I can get the info and I'll take a look later today. Thanks

I checked and I'm getting -7,-8 on the gauge at idle. Does this mean I have a vacume leak?

I'm a little confused on how this works. If your getting a reading of -8 then the PCM thinks its running lean and adds fuel to the mix. Then wouldn't the reading then become 0? If the PCM compensates wouldn't the reading always be 0? what am I missing?

Also check for any DTC's. The codes should tell you if something isn't working properly. There are so many factors that can cause a rough idle

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Dennis LaGrua: Also check for any DTC's. The codes should tell you if something isn't working properly. There are so many factors that can cause a rough idle

I'm only getting the auto-transmission code. A certain person did not disable my transmission code when I had the PCM programmed!!

quote Originally posted by My1stMidEng: I'm only getting the auto-transmission code. A certain person did not disable my transmission code when I had the PCM programmed!!

What code it that? I need to disable it, too.

Have you cleaned your MAF?

[This message has been edited by nosrac (edited 06-02-2011).]

quote Originally posted by nosrac: What code it that? I need to disable it, too. Have you cleaned your MAF?

The code is 0753 Shift Selenoid A Driver

I have cleaned the MAF