Highest Surge Ever.

[Spaz]

A few months ago this started. Worst surge ever. The engine RPM will go up and down randomly while driving or idling. When it's bad it goes up to 3000 rpms exactly and will stay there for about 20 seconds or so. If it happens when I'm driving I have to downshift a gear to keep from speeding. It seems like it's worse when it's raining or cold out, but really it happens any time. And stepping on the gas like you used to in the old days to a carb'd engine doesn't make it come down. There were no CELs but the first thing I did was check for codes. None in CM, KOEO, KOER. I'm sure my code reader works because I've used to fix codes in the past. I always heard that most surges were due to vacuum leaks so I bought a bunch of vacuum lines and replaced anything that wasn't nailed down. No difference. The car runs really well other than this problem. I hadn't done anything to the car besides drive it before the problem started. I've seen millions of threads on idle surge in all the forums. I don't remember seeing any that went up to 3000rpm. Car is mostly stock with some mild bolt-ons done a couple years ago and has no cruise control, which is kind of what it feels like. If it gets a little warmer this weekend I'll clean the IAC, although that is always recommended for an idle surge between 800-1200rpm. So the 2 things throwing me for a loop are 1: That it surges so high, to 3000rpm sometimes, and 2: That it only does it sometimes and completely randomly. I've also checked under the hood for any obvious things like disconnected lines and such. Any ideas? I'm open for suggestions before I take it to a shop. That can get expensive if it doesn't do it while it's there. It does it maybe 2 out of every 3 times I drive it. Thanks in advance.

 

[jrichker]

Codes: Did you get an 11, the system OK code?

 

[mikestang63]

first is to disconnect the IAC and see if the surging idle stops. Then disconnect the MAF and see if the surging idle stops.

 

[Noobz347]

What year is the car? Is it Speed Density or Mass Air?

 

[Spaz]

It's a '92 LX 5.0 5-speed, Mass Air. Fenderwell CAI , newer 19lb injectors, with Pro-M 76mm MAF matched to the fenderwell CAI. I got code 11 on the KOEO and KOER tests, with nothing in the "stored" memory. I just was reading my code reader book again and I think I'll do the KOER test again just for peace of mind. But I also want to disconnect the IAC when it's surging and then disconnect the MAF during the surge. Presuming for a second that it stops surging when one of those is disconnected, I guess that tell me to focus on one of those items as the culprit? Thanks again for the assistance.

 

[jrichker]

Step 1.) The symptoms you describe are a TPS or TPS wiring problem. The TPS loses its signal ground and the TPS signal voltage shoots up. This is an intermittent problem, so check the TPS wiring connector and clean the contacts. Then spread the male contact pins open to make them fatter. If that does not work, replace the TPS sensor.

If that does not fix it, see step 2 below...

Step 2.) You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 159,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

 

[Spaz]

Thanks j, I did re-run the KOER test and got a code 11. The idle stayed nice before and after the test so I didn't have a chance to unplug the IAC or MAF as someone else suggested. I'll check the TPS next. I was scrolling for quite a while on the Surging Idle Checklist and didn't see any post with a problem like I'm having, as far as a perfect 800rpm idle for stretches, and a 3k surge randomly. I also didn't realize you could post new questions to a "sticky" like that. I'll do that in the future. I'll update after I check the TPS. Thanks again.

 

[jka]

Your throttle may be sticking at times when you drive ..check your cable at the throttle body and throttle blade to see if it's sticking when you move it.

 

[jrichker]

All the first thing to check are on the first two posts of the Surging Idle Checklist. See item 2 in the checklist for the problem that most closely matches what is happening to your car...