Stewie

Here's the problem. The car idles rough at 700-1000 rpm. Any time speedo is above about 10 mph car rpm tries to go up to 3,000 rpm. I can shift thru gears but car tries to go up to 3,000 rpm. To go at 25 mph, I leave it in second gear without foot on throttle, or at 35 mph, I leave in 3rd gear and sometimes have to press brake to stay within speed limit. Then I have throttle control above 3,000 rpm. I have to brake to slow down in gear or push in clutch and car will rev at 3,000 until speedo hits 10 mph then engine drops to 700-1,000 RPMs and will rough idle. When idling, I usually smell gas and assume it is running rich.

Here's what I've done so far. New VSS, TPS, IAC valve, & MAP. Thoroughly cleaned throttle body and re-installed with new gaskets. Cleaned EGR. Checked for vacuum leaks all around throttle body and plenum. When I replaced the IAC, car ran normal for about a day.

Today, my mechanic buddy suggested disconnecting VSS. When I disconnect the VSS the car runs normal, but, of course, speedometer does not work. I'm thinking ECM even though I installed a rebuilt ECM in March 2013.

If it is ECM, I'll get one from the bone yard. I assume it doesn't make a difference if I get one from an automatic or manual transmission vehicle.

Is there something else I should check before replacing the ECM?

fasteddi

iTrader: (2)

There is a difference between the stock bin file on a automatic compared to a manual one. Id have to look though the bins to refresh my memory on the specific details.

Stewie

Thanks Fasteddi for any help you can provide. So, do you agree it is probably an ECM problem?

AM91Camaro_RS

just to be sure, not to insult you, but, is it actually running that high rpm or is the tach messed up? I know my tach reads some crazy rpm that's not right.

RBob

iTrader: (1)

There is a tach fix sticky on the Electronics board here...

RBob.

Stewie

No insult taken. Whether the tach is accurate or not, I can hear and feel the engine reving and then dropping very low as soon as the speedo hits 10 mph. Even if I am coasting, as soon as the speedo hits 10 mph, I hear and feel the engine jump in revs. If I am coasting and braking, the tach reads 3,000 & I hear it reving until I slow to 10 mph when the revs immediately drop. I disconnected the VSS wire and now it does not jump to 3,000 rpm or suddenly drop to 800 rpm when I speed up or slow down. I just don't see how the tach is the problem. The engine rev problem seems to be clearly related somehow to the speedometer. I'm not saying there is not a tach issue. But, to me, the symptoms don't point to the tach. My best guess is the ECM is screwed up when communicating with the speedo. I could be wrong and that's why I asked if I am on the right track to resolving by thinking the ECM is bad.

Rbob, I'll read the sticky to see if there is some info there that helps me solve problem.

Thanks to both of you for your suggestions. I'll report back if I find something in the sticky that resolves the problem.

RBob

iTrader: (1)

I didn't realize that it was clearly connected to the vehicle speed. That tach fix link isn't going to help.

When did this start? Was the car always a T5? Using an auto MEMCAL might be the cause of this issue. For a T5 the PROM code is BAWX.

If the PROM has AZTY on it, that is for an auto.

The PROM code is located on a silver label on the PROM itself. Should be able to read it after the small 2-screw cover is removed from the ECM.

To me it sounds like the ECM is yanking the IAC open which is why the RPM increases.

RBob.

Stewie

RBob - The car has always been a T5 and the strange behavior just started one day. I could not pull any codes. I changed the IAC and it stopped doing it for about a day. Then I changed the TPS which made no difference. I removed and thoroughly cleaned the throttle body and EGR. After putting everything back together, I thoroughly checked for vacuum leaks. Same problem. I replaced the MAP. Same problem.

The SES light would come on intermittenly, but I could not get any codes. Finally, I left the engine running when the SES light was on and got a code 24. That's when I replaced the VSS. However, I later found that I should have had it in gear with the wheels turning to get an accurate code 24. At any rate, a new VSS didn't stop the problem.

The only way to stop it from jumping revs between 800 and 3,000 was disconnecting the VSS wire.

I did replace the ECM with a rebuild in March 2013, but used the PROM that has always been in the car. The PROM came with the car when I bought it in 2007. If it was the wrong PROM, I'd think the engine would have acted up sooner.

After trying everything, I gave up and had my busy mechanic buddy check out everything and said he is not positive, but thinks I've pretty much eliminated everything but the ECM. He said "the problem is that car computer tattles on everyone but itself." I should have asked for his help sooner, but I'm a bit bull-headed and over confident.

If I you have any other idea of something else to check first, I'm willing to try anything. But, it sounds like you also are leaning toward it being a bad ECM.

However, if I replace the ECM and have the same symptoms, I'll probably drive the car off a cliff; at 3,000 RPMs, no doubt.

RBob

iTrader: (1)

One other thing you can try, is to warm up the engine and unplug the IAC (electrical connector). Drive the car and see if it stills does it. If not then we'll know that for some reason the ECM is opening the IAC.

Won't pin point the bad part, just a bit more information.

RBob.

Stewie

Thanks. I'll try it.

Stewie

I'm back & getting ready to drive my car off a cliff at 3,000 RPMs.

To recap, car runs at 900-1000 RPMs at idle & jumps to 3,000 RPMs above 10 MPH and will NOT drop from 3,000 until speedometer hits 10 MPH. So far, replaced TPS, IAC, VSS, and MAP. Thoroughly, cleaned throttle body and EGR and re-installed both with new gaskets. Thoroughly checked for vacuum leaks. When I disconnect VSS, problem disappears.

Gotta be a bad ECM. NOPE. Replaced ECM today & same problem. Used PROM (code ARPL) from old ECM. Neighbor mechanic saw me struggling and offered to help.

One by one, we disconnected wires to IAC, EGR, and MAP and checked codes after each and then cleared codes after each. Same problem. Tried different MAP in case old ECM damaged. No change. Neighbor has electronic code checker that works on GM.

Pulled off TPS and re-installed. Also re-adjusted TPS several times. No change. Really appreciate my neighbor spending several hours trying to track down problem. He even offered to continue helping tomorrow after he completes repairs for his paying customers.

I have a used throttle body with TPS & IAC that I'll install tomorrow. If engine runs the same, we can rule out anything directly related to those parts. Neighbor also found connector with a lot of wires near firewall & blower motor which had some gunk. Suggested I clean with electrical cleaner which I'll do. He also thinks there could be electrical short in wire to ECM.

Any other ideas? Could a bad PROM do this? Thanks for any suggestions.

Stewie

Tried after market PROM. Same problem, but jumped to 2,500 RPMs instead of 3,000. Dropped to 1,100 immediately when car slowed to 10 MPH.

Stewie

This is not going to be one of those threads where you don't get to find out how the problem was resolved.

I kept driving my bird with the VSS disconnected and no speedometer. I used the tachometer to monitor my speed and evade speeding tickets in between trying to figure out what was wrong. Finally, my mechanic buddy said "leave it with me and I'll keep working on it in my spare time." That was 3 months ago. He got sick for about a month and then had to work his butt off to catch up on the paying jobs. He's spent the last month tracking down electrical, disconnecting individual parts, trying to find the problem. He's had it so long my GF jokingly asked if he wanted me to just give him the title. :-)

Earlier this week, when I was handing him a jug of gas because he ran out of gas on a main street IN MY CAR, lighting struck my brain. I said the only thing I haven't changed is the coolant temperature sensor (CTS). I said I didn't know if the speedo & CTS are on the same circuit. I'd also picked up some used parts including a complete throttle body with attached TPS & IAC.

He called me this evening and said it's fixed. But, he apologized for installing the complete used throttle body and CTS all at once rather than one at a time to see which was the problem. I'm convinced it was the CTS, because he'd already disconnected everything else individually trying to track down the problem.

Bottom line, I think a bad CTS was the problem. Just can't be completely sure. But, if anyone else has similar symptoms, CHANGE THE $11 CTS FIRST!