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A Rough Running Caddy

Q. I've got a 1990 Cadillac Eldorado, V-8, fuel injected, 130,000 miles. Recent tune-up (within 4,000 miles) been to two shops w/problem - neither can identify/solve. I'm reluctant to go to the local dealer. Last time I went there it took $1000 in repairs for them to troubleshoot and replace a faulty fuel pump.

Immediately after starting and continuing for as long as 20 minutes, regardless of "hot" or "cold" engine, regardless of simply idling, city or freeway driving, the engine seems to run on 6 or 7 cylinders. Poor performance, guzzles gas (per "instant" mpg on elex info center readout), bucks, chugs. No pings or obvious valve noise. At idle, exhaust smells of unburnt fuel.

After a period of time, engine magically begins to run smoothly. If on the freeway, "instant" MPG picks up from 14 to 23 MPG, car no longer downshifts to accelerate, no more chugging or bogging down. Turn off the key and the whole problem repeats itself. The problem appears unrelated to engine or coolant temp and doesn't again occur - until I turn off the ignition and re-start the engine.

I suspect some sort of time/thermo switch/sensor in the fuel injection......

If this were my '73 beetle, I'd say the automatic choke was set too rich.

Thanks for your help!!

A. Drivabilty problems are always hard to diagnose, especially ones like this. My first thought is a bad Throttle Position Sensor (TPS), but that should throw a code and light up the MIL on the instrument panel. Since you didn't mention the check engine light is on when the problem occurs, I'm going to assume it's okay at this time. The computer will report a malfunction in most of the fuel injection components, the TPS being one of them.

A TPS is easy enough to check, so here's how to do it. You'll need an ohmmeter for this test and maybe somebody to move the throttle for you. This is a case where an analog (needle) type meter is better than a digital meter. Locate the TPS, it will be mounted on one end of the throttle plate shaft. With the engine off, unplug the three wire connector and lay it aside. On the TPS itself place one meter lead on the center pin and the other lead to either one of the other pins. One side should show an open (no needle movement) and the other should show a closed circuit (needle jumps). If both sides show open, the TPS is bad or out of adjustment.

If it checks out we procede with the next step. With the throttle halfway open repeat the test, this time you should have an open on both sides. If not, the TPS is no good. Again, if it checks out, we procede to the next step. With the throttle wide open, repeat the test, this time one side should read open and the other side should read closed. The results should be opposite of the test we did with the throttle closed. In other words if A and B showed closed at idle, B and C should read closed at wide open. at no time should there be continuity between A and C. If there is, the TPS is no good.

Another possible culprit is the Air Flow Meter (AFM), Oxygen Sensor (02) and the MAP sensor. Any of these components will also cause the same symptoms. Again, these should throw a code and light the MIL. Another good possibility is a bad EFI computer. We have a saying; "if the rest of the system checks out okay, then it's the computer." That's because you can check everything but the computer.

If two shops checked out the problem and were unable to come up with a cause, then a Cadillac dealer is your only recourse. They will be able to come up with the answer. They will locate the problem and give you an estimate on what it will cost to repair. If you feel the price is too high, you are well within your rights to take it to another shop and have the diagnose confirmed and possibly have the same repair done for less.

Don't begrudge the cost of the troubleshooting, problems like this take time to find and two to three hours diagnostic time is not uncommon. GM trains their technicians well and the dealer is the one who pays for that training. The equipment GM requires the dealers to use to locate problems like this are expensive, again paid for by the dealer. There is also a support system in place where GM can access a database of thousands of cars with symptoms like yours and what it took to repair them.

I hope you can get this fixed, it is a most annoying problem.

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Additional Information provided courtesy of AllDATA and Warranty Direct
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