Questions and Answers
A Rough Running Caddy - Revisited
Q. Vincent, Reference your Q&A: Questions and Answers: A Rough Running Caddy. I had what seems like that exact problem on my 1993 Deville when it had about 75,000 miles on it. No trouble codes were set, no check engine light was on.
The local dealer first said it was acting like it needed new plugs, wires, cap, etc. What with the age and mileage, I couldn't disagree. The "etc." was about everything else in a total tune-up, bringing the bill to almost $600.00.
But that didn't help it. $600.00 down the tubes! The problem finally was found to be intermittent injector drivers in the engine computer. R&R the engine computer, and all is well. A Delco 'exchange' computer for this car is only about $200 and they didn't charge me any labor for replacing it. (I guess they did have SOME shame!)
What is most interesting is that it never set a trouble code. Seems that when the engine is cold and the O2 sensor is out of the circuit there is no feedback for fuel mixture; and when the O2 sensor gets hot and starts working, there is no trouble code for intermittent mixture error? Weird!
One thing worth remembering is that for problems which are intermittent but do not seem to be temperature related, the engine computer is inside the passenger compartment and is thus insulated from physical engine temperature (and ambient temperature too, somewhat).
A. In the older diagnostic systems a failure had to be a fairly "hard" one to set a code. So quite often an intermittent problem will not trip the computer into setting a DTC. Several auto makers came up with a "real time" mode on their diagnostic tools that were far more sensitive in picking up intermittent problems. I know Nissan had their "Mode Five" on their tester that would display a code the moment an intermittent showed up.
Also there were no provisions for testing the ECU itself. It was assumed that if everything else was good, the ECU was bad. An imperfect assumption that caused a lot of ECUs to be replaced needlessly. This is the main reason I have test ECUs to verify the quality of the customers ECU and prevent needless replacements and still not repair the problem. Another interesting fact is that a failed ECU is pretty low on the list of most common failures.
Additional Information provided courtesy of ALLDATA
