I have been having a strange issue with my 90 STS for the past few weeks. When I start it up, it revs very fast for the first second, probably close to 3000 rpm, and than immediately comes down to normal idle speed. When I am taking off on flat ground, I just tap the pedal very light and it starts rolling about 5 mph, but than I let my foot off completely it very slowly starts getting faster as the engine slowly speeds up until the it gets to second gear.

Once I am up to over 20mph the problem doesnt really matter, but than when I slow down and I am just about to stop, the engine keeps pushing the car. It's not a hard push, but the engine is definitely working against my brake pressure. At this point the car is in second gear, and it only downshifts back into first gear at the very moment before I am completely stopped. It's kind of a feeling like if I had pressure on the brake but was pushing the gas at the same time.

The transmission shifts beutifully (140K) and the engine runs like a top, so I am pretty sure it is an electrical thing. My only guess was maybe the throttle position sensor was bad, but I wanted to find out if anybody else had an issue like this with their 90 4.5 Caddy. I also had this issue once about a year ago out of the blue, but it went away after about 4 days. This time it has been going on for about 2 months. I can live with it, but it would certainly be nice to fix it if it is something easy.
Steve

 

What happens if you tap the Mass Air Flow sensor (MAF) hard with, say, the handle of a screwdriver, while the engine is idling? Dont break it, but rap it good. A change in idle when you do this tap-test indicates a bad MAF. In any case something is causing the engine control computer (ECM) to increase fuel flow, and this is usually because of an increase in air flow, or at least the computer thinks it sees an increase in air flow. Can be caused by a vacuum leak, bad MAF, air intake leak downstream of the MAF, bad O2 sensor, loose fuel injector harness connections, or a defectiveECM. That is to say that it could be any number of things, but my experience is that the ECMis getting a faulty air flow reading somehow so it is boosting fuel input, thus increasing engine speed. Do you get a service engine light and can you pull codes?
Keith
1995 SLS

 

Yes, the light is on. The only code I recall is E049 which I think is for the AIR stuff, but I'm sure there were some others. I will be away from the car for the next week, but I will do that test when I get back to it.
Thanks for the input,
Steve

 

Oh yeah, an emissions problemcould do it too, although I'm not sure how.
Keith

 

This car has NO mass airflow sensor!! Sounds like you need to verify that the TPS ( throttle position sensor) is correctly calibrated. If base voltage is too high it will do what you describe. Also, the ISC ( idle speed motor ) must be verified as having the proper clearance as well. The TPS monitors the throttle angle at the throttle body and based on that, delivers more or less fuel. My 91 4.5 Allante was doing that and we discovered the TPS retaining screws were loose and because of that, the TPS had advanced the base voltage reading. The TPS starts at around .5 volts at idle and can go as high as 5 volts at wide open throttle. If its at 1 volt instead of .5 it will not return to correct idle either at all or very very slowly. A quick and dirty way to know if TPS is wrong is to monitor instant fuel economy while driven above 45 to 55 MPH. If correct, you will see a display of 70 MPG coasting , foot off throttle pedal. If lower than that, more than likely TPS needs adjustment.

 

Thanks, I'll give that a shot next weekend. I went on a trip this week and decided to take my 90 ETC. Now the tranny in that is starting to die. If its not one thing it's something else! lol
Steve

 

Well I got back to my STS last weekend only to find that there is once again no problem. Whatever glitch there was seems to have corrected itself, as it is starting up just how it should, and there are no codes. I did the check of the instant fuel economy at 50 MPH and it read 70, and all the sensors etc on the throttle body were nice and tight. I'm not going to argue with it now, but I sure hope it doesn't act up again! On another note, I drove back to NY this weekend, 350 miles, and averaged 26.2 MPG (my calculation, not the computer).

Steve