Cadillac runaway acceleration

This is what I don't get either. In cars of this vintage there's only so much the computer can physically do. It doesn't control the throttle except maybe via the cruise control. That's easy enough to render impossible by disconnecting the cable from the cruise control to the throttle.

Solenoids are on or off. An idle Solenoid is going to open an air passageway just large enough to flow enough air for the engine to idle.

The computer does control fuel and it can dump in as much as it wants but without air that's not going to do a lot of good.

I think the route to solving this is to find where the air is coming from.

I would disconnect the cable to the throttle from the cruise control if it hasn't already been done and tie it up somewhere safe. Make sure the throttle cable works properly. Same with the throttle body. Then I would look for cracks in the intake hose, throttle body gaskets, intake manifold, etc. If none can be found visually then a smoke test. See where the smoke comes out and that's the problem the that's throwing the computer off or compounding with another issue. It's simple enough to make a home made smoker I learned by making one if anyone cares to.

The engine has to get air from somewhere to run away like this. foot on the throttle. throttle cable sticking open. cruise control opening the throttle. leak path. Something.

No cruise and it wasn't running rich. It ran much faster than high idle, and you had to use firm pressure on the brakes to keep it from taking off. When it happened to me I pulled into a parking lot, put it in park and checked the TB linkage. It was ok, but the engine was going 2000 RPM or thereabouts. I considered replacing the TB but took it in within a day or two since I was working a lot of hours. I had a real good mech, who prided himself for not throwing parts. The TB would have been a waste of money. I've got no idea how the ECU could rev the engine to @2000 RPM. It's beyond my knowledge. Those old ECU's were prone to failing from heat IME. My '88 Celebrity's failed too, but showed different symptoms. I took it in when it was killing the engine on the highway. Since I was going about 70 mph it instantly restarted, but slammed the trans. I thought it was the trans. The trans guy said it was okay and my mech said it was probably the ECU shutting it down. I left it with him and he kept it a week using it as the shop car, wired with a portable analyser. Unfortunately the day I dropped it off the weather turned cold and they got nothing from it. When I picked it up he wouldn't take any payment. He said "I'll get you later." It ran perfectly until the first warm day in the spring, when it wouldn't start. No spark at all. He got me, but it wasn't bad. About 2 bills. That was my favorite car of all time, mechanically. 2.8. But it peeled paint and rusted out by 2001. I

Right. And THAT is why I am thinking of mechanical issues. Cruise control. Stuck cable or dirty throttle bore making the butterfly stick. Or the interconnection with the transmission.

I can't help but that that if it's intermittent like this that it's coming from the throttle being open. I'm not saying there is no crack anywhere or a bad gasket but the pattern doesn't seem to make that likely.

But if air DOES get into there.... the computer will do what it's supposed to be doing and throw fuel in there to burn with it.

I am inclined to agree.

--scott

Guaranteed crash. It's roughly two feet under the windshield with dim green letters about 3/8" high and it takes my eyes at least a second to adjust to the darkness and closeness if I'm lucky. AND if it's already set to RPM rather than engine temp or battery voltage. I can't imagine anybody signing off on such a stupid design.

Next time I'm on the freeway with a passenger I'll floor it and have him see what it reads. FWIW, I did notice today that slow idle=550 and fast idle=750

I think simultaneous is probably essential. Not impossible. Dirtriders can brake with both hand and foot and use the clutch and throttle and foot-shift at the same time while standing on the pegs in a pinch :-)

Nobody ever said it works all the time, though.

Well, I wouldn't take it to the dragstrip but I've never had trouble entering freeways or passing slower cars quickly. Yes, I've had cars that were troublesome in both departments :-(

This week is devoted to cataract surgery. When I have binocular vision again I'll go back to the guy and see what he says.

I wish I'd laid this on Click and Clack a few years ago. I had a weird problem with the Sentra that happened only at a certain place +/- a couple of miles or so, and only when I was going skiing early in the morning. It would just die, like it ran out of gas. If I waited a while it would eventually start and I could continue.

Somebody else had a similar problem -- a stuck-open air-intake butterfly thingy that allowed really cold moist air to blow in and freeze up somewhere serious. Waiting allowed it to melt, and by the time I got going again the outside air had warmed up or dried out so it was no longer a problem.

Well, I had the wrong car in mind anyhow. Those actually did run pretty good. I had the earlier body style & engine in mind. They were really awful.

I did want to add one more thing. I know you said the cruise was disconnected years ago, but are you sure some well intentioned tech didn't reconnect it on a more recent trip to the shop?

Except for the cheesy, useless and easily-breakable "features", it's a fine car for a stupidly-designed piece of shit. I'd never buy another one, or even take one for free unless I was really desperate, though. I really wish my mom had asked my advice when she bought it. She would have been happier with a Lexus or something (she probably thought I would try to talk her into a Corvette).

Nope, he's the only one who's touched it seriously. I doubt the Pep Boys oil-change guys would do it. Still, I'll ask him.

Two data points:

(1) On the freeway, at roughly 55 mph, I gunned the engine. Passenger reported it went up to 3K before I backed off, at which point it dropped normally. Just a test...

(2) Leaving a parking lot, with my foot on the brake, the involuntary acceleration started. I immediately shifted to neutral and watched the rpm rise to 2700 before I tapped the accelerator, bringing it back to normal.

Curiouser and curiouser...

2700 is about the limit of the ISC motor. The contact switches in those can go bad, and cause that (it doesn't know it's in contact with the throttle lever), or the ECM. I would think that if it was the ECM, it would happen more often, but the tap test might help, too.

Regarding the ISC, the plunger is spring loaded, and you can feel the contact switch with slight pressure - it will move very slightly in and out. That switch could be bad, but another cause is weak throttle return springs that can't push the plunger in enough to close the contacts, or a sticking throttle. In fact, either of those two problems can cause what might feel like unintended acceleration.

If this happens again, you can always put it in park, open the hood, and look to see if the ISC is fully extended, holding the throttle open. If it is, push the plunger in, and if it retracts then, it's almost for sure a bad ISC motor, unless you can see that the throttle lever is clearly not pushing against that plunger. If that's the case, it's either the springs, or a binding throttle shaft, or throttle plates.

*I* backed off; the rpm was still rising in both cases.

Not "feel like." IS!

...for extremely rare values of 'always'.

Next week I go back to the mech with a lot of questions...

The bad news, which was ameliorated before by replacing the return spring with a heavier one: bad bushings in the throttle body, allowing sticking, slop, etc. No replacement possible, unless maybe from a wreck. The AC is shot, along with the shocks. The front tires are 12 years old. The headliner is held up with long ball-headed pins to keep it from flapping. The driver-side window will only go down 6" or so. Lots of plastic bits are just broken.

I truly wish my mom had asked us before buying this POS. A "luxury" car with 85,000 miles on the clock should NOT need this much crap in addition to the crap it's already demanded.

Yes, that was a common problem, and years ago. I don't know if I remembered to mention that one, though.

There might be some specialty shops that can replace the bushings, or there might be aftermarket throttle bodies somewhere. At some point, though, it's not worth the expense.

He checked his aftermarket sources. If it was possible to find a wrecked unit from a wreck that happened 20 years ago I'd be OK with that, but apparently you have to replace injectors too.

I'm looking at several $thousand in AC and shock replacement in addition to this plus comfort-only stuff. I don't think so.

There's where the air is getting in causing the increasing speed/rpm. It depends on what the bushings look like but they can probably be easily made, found generically as hardware, or removed from any number of GM cars.

Shocks get expensive with someone else replacing them.

GM has been using headliner glue that lets go after 15 years for 30+ years.

regulator issue. Probably a broken gear. A plastic gear knowing GM.

With the issues you're talking about miles aren't the problem but years. It's all materials degrading over time and exposure to the elements.

For $8K you can buy a nice one owner one.... Always garaged...

Bingo. That's exactly the sort of thing I was suggesting and the reason why I was suggesting playing with the throttle by hand.

Take the throttle body off (you can do it yourself, it's not a big deal) and take it to a machine shop. Have them see if they can rebush it. They'll have to cut new bushings on a lathe out of oilite stock, but it's not really all that difficult.

GM quality has improved a lot since this car was made. It hasn't improved enough, mind you, but things are much better today.

--scott

I know, but I just don't have the mental energy or painless back required for the job any more. Long ago I thought it would be cool to have a grease pit in the back yard. I still think it would be cool, but not having one is an excellent reason to pay somebody else to do something :-)

GM is evil. Not stupid, evil. Not just planned obsolescence, planned destruction.

Probably. The 1960 Ford broke the little plastic roller that pushed on the window to roll it up and down. 25-cent part, taking the shit off the door was the troublesome part. Probably the same here, except for the gear-grinding noise at the top and bottom. More crap. Oh yeah, the interior door-puller broke so I have to close the door by hooking my fingers into the space between the window and its frame.

The paint is in fair condition, though :-(

Thanks, I think I'll skip that one.

We could probably do that ourselves, we have a little 5" lathe that hasn't moved for 40 years. Not gonna do it, though. The car is at the point where stuff is just going to start breaking and I don't want to deal with it any more. I really am superstitious about this -- fix one thing and the car will deliberately break something else.

Never again. The 73 Dodge pickup is made of real metal. It goes, it stops (well, it needs some serious brake work, but it stops if you pump), and its problems are understandable by humans.