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Hello, I am in some very big trouble. I have a 88 z24 cavalier and when I shift gears or push in the clutch, the motor goes to 7000 rpms.Car idles fine. It seems as though it gets worse the longer I drive it. I have had the throttle sensor checked, it was ok. I checked for vacuum leaks, seen nothing. I did do a code check on it, came back code 24. Which was speed sensor, but said disregard if car was not in motion and said check throttle sensor. I had advance auto look up a speed sensor when i had my throttle sensor checked. Nothing on file for a speed sensor for a manuel transmission. I did put a new clutch with pressure plate in it this past summer. I'm out of ideas. Please any help would be appreciated.. Thanks, Shawn
Answer -
Hi Shawn,
I would look at the throttle plate to see if it is actually open.
I don't know if that is cable operated, which could stick, or electronic. But the engine can't run without air, and it is either getting it from the idle air control valve, or the throttle plate, or a big vacuum leak, which could be the brake booster, or the EGR valve. But I would sure suspect the throttle plate is open, cause that is where it would get the signal for more fuel also.
That throttle sensor could be just fine, and showing that the throttle plate is open.
Van
Van,
Im seeing on the cavalier website, it says check SETTINGS at tps with code 24, what are the settings? also, it is cable operated throttle. Is there anyway to check the EGR valve and the idle air sensor to see if its any good? Im sorry man, im in a pickle here and not really sure where to start now. Thanks for the help, Shawn
AnswerHi Shawn,
First, make sure the cable isn't sticking, and that the throttle plate is actually closing. The position of the throttle plate is what the throttle position sensor goes by.
As for the settings, some, but not all, throttle position sensors have slotted mounting holes. They don't allow a great deal of adjustment, but a little bit goes a long way.
The egr valve can be checked, and should be removed and cleaned. if there are carbon pieces coming loose occasionally inside the engine, the pintle of the egr valve can close on one, and it will hold it open. That will act like a big vacuum leak.
The idle air is not a sensor, it is an idle air control valve. When activated by the ecm, it actually opens a port, and allows air to flow around the throttle plate, in order to have enough air to idle when the throttle plate is closed. Thus you realize, the throttle plate must be able to close completely, so see if it is.
If you unplug the idle air control valve, and then reconnect it and turn the key to run, it will cycle a little as it closes all the way to find out where it is, and then opens.
It also should be cleaned, and can be removed and hosed down with carb/PCV valve cleaner, or cleaned in place. It usually screws into the front or side of the throttle body, and the port should be cleaned also.
Look for other vacuum leaks also, like the base gasket of the throttle assembly, the egr valve, the brake booster, the pcv valve, the intake manifold gaskets, the heater controls if vacuum operated, and all the vacuum hoses. If the ecm gets a lean reading from the oxygen sensor, due to extra air, it can try to compensate for it by adding fuel, which can make the idle search.
A bad temperature sensor is also a possibility, since it is looked at regularly during the operation, and mixture decision of the ecm.
See this link for some discussion about that, as well as a chart for what the readings should be.
http://www.chevytalk.org/threads/showthreaded.php?Cat=0&Number=1366279
And then there is the mass airflow sensor, which can get dirty, and cause wierd operation. The best article I have seen on that is from a Ford site, but the sensors work the same, and the following link can be very helpful.
http://www.fordscorpio.co.uk/cleanmaf.htm
Good luck,
Van
