Cadillac runaway acceleration

What model car is this? Does it have a carb, or fuel injection? I am assuming if it's older than 2007 it's not drive by wire but has a mechanical coupling from the throttle body to the pedal.

Next time it happens, slam on the gas pedal and see if it frees up.

That's not the problem but it certainly can't hurt to replace if it is surging.

Did he take the throttle apart and grease it, also look for any signs of wear?

The position sensor is a red herring if it's an older car but might be a good try if it's a newer car.

The cruise control would have been my first guess, actually. The smog pump, belt, and engine mount are unrelated but probably normal routine maintenance.

How did he disconnect it? Did he physically disconnect the linkage to the servo?

Assuming this is a fuel injected car and it's not drive by wire, it is likely that the throttle is just sticking open. I hope that he cleaned the throttle body at some point and greased everything... that would be a half-hour job and a possible source of trouble.

I hope he had an assistant pump the gas pedal with the engine off, and looked at the throttle valve to see if it was opening and closing properly. I hope he also went looking with a propane torch or a smoke machine for any kind of leak into the throttle body.

It is likely that the valve is sticking open, which could be because the valve is dirty, or because the cable is bad, or because the pedal mechanism is bad. If it happens seldom it will be hard to replicate but on the ground you can work the pedal back and forth many times to increase your chances.

What kind of engine is it and what year? The odds are that the throttle is entirely mechanical and the computer is just following the throttle rather than controlling it. But without knowing what kind of car it is, it's hard to say.

Also, if this is an automatic transmission car, there may be linkages so the transmission can control the throttle, and those might be suspicious too. All depends on what kind of car it is.

That's curious. That makes me think maybe you have a 1970s car with a carb.

Get the dealer mechanic to pull all the ECOs (or whatever GM calls the engineering change tickets) on the car and see if any of them are related.

--scott

How can you expect advice when you don't give the year and engine of the car? But it's most likely the computer or your feet causing it. I had the computer problem with an '85 Cavalier. New computer fixed it. And the foot problem once with a '93 Corsica when my son put big pads on the pedals - his car. Not driving his car ever again fixed that.

...

Feet = no: I told my passengers (1 husband, 1 friend) LOOK, MY FOOT IS OFF THE PEDAL and raised it up so they could see before I hit the brake. I was getting good at it by then.

Computer? More and more likely.

Some lessons are harder than others.

Sorry, I didn't copy that over. 1988 Eldorado with fuel injection.

Interesting. I'll try that, but I don't think that's it.

A surge is a sudden thing that relapses; this is like a full-throttle stomp on the accelerator which doesn't stop until I turn the key.

I don't know. I would think that he was curious about why/how this was happening and would have done whatever he could. He's been extremely good about diagnosing weird stuff in my friend's Cads.

They were just on that bill and I left them in for completeness.

I don't know. My guess would be that he did whatever would be most likely to prevent it coming on. By this time it had happened several times.

1988 Eldorado. 4.5L V8 like all of them.

Fuel injection.

I chatted with the dealer's tech manager about this. He looked up something and said there were no factory . No recalls either. He wanted $250 with no guarantees to check it out. I have reason NOT to trust the local dealers so didn't take him up on his offer.

To be more specific, the Cavalier would occasionally rev up and stay up until the ignition was shut off. Wife's car. My wife complained about it twice, but I thought she was exaggerating. Then I was driving it when it happened. It would go about 45-50 mph with my foot off the gas. Took it to my mechanic, who replaced the ECU. Can't remember if he used a new or boneyard ECU. Car was a beater. 2.0 TB injection. Your ECU probably runs $55-87. I don't know the labor cost.

I'm assuming that this is the Deville, and not the Brougham, and I saw in your later post that it's a 1988.

Going towards the end of this post, if the problem stops when you tap the brake, it's almost certainly the cruise control turning on, and you might be doing that inadvertently. Are you completely familiar with how to work the cruise? Either way, it's a simple matter to just disconnect the cruise linkage from the throttle body. If that eliminates the problem, you know it's a cruise issue.

Regarding the earlier issues in the post, those idle speed controls (ISC's) would extend whenever the engine would start to falter, or come close to stalling, and that might have been what you felt. Or it could have been a bad speed control doing the same thing - extending. It's very possible you had two problems, and replacing that control fixed the one, and the only remaining one is cruise related.

There is really nothing the computer can do to cause this, other than extending the idle control. The ISC can extend only so far. It might scare you at first, but the car is not going to get out of control - it gets nowhere near flooring the accelerator.

It's also possible that the pedal was getting stuck under the floor mat.

Anyway, if this is a Brougham, I think those were still carbureted, but all of the above should still apply, but the idle speed might be controlled differently.

If you Google "1988 Cadillac unintended acceleration", you'll get a few hits. I couldn't find anything with the diagnosis, but I'm pretty sure they were all ISC problems. It's also possible there were updated PROM's for the computers, but if there were, a car that old should have had that done long ago.

I forgot to add the possibility of a bad ECM. There were lots of issues back then. It's still the same issue - extending the ISC.

Eldorado.

The cruise control hasn't worked since before 2005 and was disconnected the third time I took it to the mech.

Uh. Do you think I'm lying about this? I've been driving cars since

1957 and motorcycles (street and dirt) since 1965. I know what full throttle means and feels like. Honest.

Yeah, right. And turning off the engine somehow magically changes this...

Not to be snotty or anything, but it's pretty damn insulting that you mentioned that.

Generally ISC, and that was the first thing the mech did. Mom had the beast serviced regularly every 3 months by the s*****ad local Cadillac dealer, and I assume that if he could have charged her for updating the PROMs he would have done so. I nearly cried when I looked at her receipts.

ECU = ECM? I looked at some stuff about those. Idle was mentioned, but not full throttle. Still, I'll talk to the mech about it.

This is NOT high idle.

Yes.

It's not full throttle either. Nothing can make it go to full throttle on its own. I believe the ISC's when fully extended drove the RPM to about 2500. Cruise could theoretically do it, but you say it's disconnected. If it's at full throttle, you pushed it there, and it stuck.

Those cars are also sadly underpowered, and there really isn't much difference between part and full throttle.

Same thing for our purposes.

Was it properly disconnected? Honestly, if tapping the brake causes the throttle to release, I can't think of anything else that would be the cause.

Like I said in the other response, with that underpowered car, there is little difference between aggressive part throttle, and full throttle.

Sorry, but it's not impossible. Pedals caught under a map can release on their own, and could coincidentally release when you turn off the key. Coincidences have led me down many wrong paths, but I finally learned to stop eliminating things based on what seemed at the time like logic.

It's not surprising, or necessarily dishonest. Those cars were very, very, hard to properly diagnose in the earlier days of computer control, and a lot of guesswork went on. Even calling the factory got us answers like, "change everything one at a time until the problem goes away". And no, I'm not kidding.

Also, when that ISC was replaced, adjustments are needed. And with that, and any electronics (ECM, PROM), an idle learn procedure is needed, or you end up with an unstable idle.

Was it running super rich when this happened? And was it equipped with cruise control?

I can think of the ECU doing three things here: maybe it activated the cruise control servo, or it activated the idle solenoid, or it just went super rich with that injector spraying heavily. That third one is more likely to make it stall out than race, though.

--scott

The idle control system in an injected car replaces the idle mechanism in the carb, it opens the throttle up a little or injects some air into the intake around the throttle to keep the car from stalling out when your foot is off the gas. It can theoretically cause trouble if it comes on when the car isn't at idle. You've already replaced the valve but the ECU (or ECM or whatever GM calls it this week) and the wiring may be suspect.

I'm still thinking of mechanical issues, though. I'd really want to make sure the throttle body is clean and the throttle moving smoothly and there is nothing to catch on the linkages before I did anything else.

--scott

With an older car, you could pretty much check everything mechanically and find that any failure was due to something being out of tolerance or leaking.

With a newer car, there is a lot more going on, but you can plug a computer terminal in and not only look for errors logged by the ECU, but you can also look at all the signals from all the sensors going into the ECU and all the signals coming out of the ECU to control things.

Unfortunately... this is a car that is in the dead area between those two technologies. There is an ECU, but it has only minimal error reporting, and so there is little to be done about it other than to treat it as a black box and replace it. Of course, that might not fix anything, but it'll let you know it doesn't.

Is that the case for the 1988? I think back then the ECU was not stateful, but it might be new enough to hold state information when turned off. I am much more familiar with the Bosch stuff than GM.

--scott

Probably not. It sounded and felt OK and got the usual gas mileage. Cruise control stopped working in maybe 2005 and was completely disconnected after the third incident.

Is the "powertrain computer" (inspected via the 'tap' method) the same as the ECU/ECM?

That's standard procedure for a lot of things. It would be nice if you didn't get charged for the things that didn't work, but those guys gotta live too.

The bastards (Caddy dealership) sold her obsolete tires and then referred us to Bridgestone when they developed bubbles. They replaced all the hoses and belts and (I think) vacuum tubes at perhaps 30K miles. Other stuff that I thought wouldn't have been needed, but the papers are all gone now. OTOH, they picked up and delivered the car and washed it. They're out of business now, possibly because the mostly-Asian community Alhambra became wanted to buy Asian cars and possibly because they were just crooks.

Pep Boys pulled a few codes before this started, but they were hard to decipher and irrelevant unless they were actual errors. The mech said that if the problem occurred due to a computer reboot the error codes would have been lost. Feh.

Idle is fine and always has been. No starting or running problems, everything sounds OK.

Do cars still need "tuneups"? The only things I've done to the car since 2007 were oil changes every year (or 3K miles, generally the same), new battery, smog crap every two years, a new transmission oil pan when I drove over a rock, a new motor mount and serpentine belt, and stuff related to this problem. No new plugs or anything else. If it ain't broke don't fix it.

Sudden acceleration when I've been driving 65-75 mph steadily on the freeway for minutes or 40-45 uphill on a mountain road can be caused by some idle weirdness?

No. Honest, no. I don't know why Cadillac even bothers to put a tach in their electronic console -- you can't actually see it while you're driving in sunlight, and I'm more concerned about engine temperature anyway. That console is another stupid design "feature" that should have been axed by management as soon as it was suggested.

Actually, it wasn't. I was impressed with the ACTUAL full-throttle acceleration when I started driving it. It's not as good as an S2000 or a Suzuki GS550, of course, but plenty good.

I don't like to stress cars or motorcycles unless I have to. I am a conservative driver (except in my friend's S2000 on a mountain road). I am reporting what happened as accurately as possible and the S2000 friend -- who also repaired her own cars until she could afford to have someone else do it -- is a witness to one incident.

That one does. Plugs, filters, etc.

I'm pretty sure those still needed the idle learn.