Hello all,
Vehicle: 1990 c1500 with a 350 tbi/700r4
Problem:
High idle at cold start (around 2000RPM) High idle when warmed up (around 900-1100rpm) Slightly rough idle when warm. Performance otherwise fine, no stumbling off idle or missing under load or revving in neutral. Drives great. Only a slightly rough idle that's too high and creates minor vibrations in the cabin when in gear at a stop light for instance. That's annoying.
Idles around 700-850rpm in gear.
As it does have a higher idle cold I would assume the CTS and IAC are working.
My guages all seem to read low. My voltage at the battery is 14.3v (new battery and alternator) but my gauge in dash reads 10-11v. Coolant temp on gauge reads 150 on average. Heater is pretty warm though. Seems ok. My oil, coolant and volt meters decrease in rythm with my blinker, I think this is a seperate cluster issue with grounds but worth mentioning.
History:
The PO tapped the idle adjustment screw and had obviously messed with it to compensate for this rough idle I'm assuming.
First thing I did was back the screw out a bit, this helped lower idle speed but the lower RPM did worsen the condition of the Idle. But I wanted less violonce when engaging trans.
So far I've:
I've changed the plugs, gapped at 0.35 which helped the idle slightly, but it's still not smooth. It has slight shudder every so often at idle when warm. Cold it's idling too high to shudder.
Pulled the cap and rotor and found crud on all the terminals inside the cap. Cleaned them all and sanded them lightly. Rotor too.
I've replaced the IAC with a used one I had lying around. This has changed nothing although the old one seemed to be pretty stiff (should you be able to push it in and out by hand?) and was very dirty. Trying to push the plunger back and forth it snapped. Should these be unmovable?
Either way, 'new' (known to be good used) IAC installed: No change.
I noticed when trying to reset idle speed accoriding to procedure (ALDL ports A-B jumped, ign. off, pull plug on IAC, start engine and adjust to 650rpm) that I can't get it too idle under 2000rpm with the IAC unplugged! That's the lowest, at that point the idle screw is fully backed out and the butterflies can't go back further. Also interestingly, the system is compensating. It adjusted to this lowest setting. Before the throttle was held open a little by the idle screw, now it's not touching and at first it died now it runs the same as before (described above) so it seems the IAC is doing it's job of metering idle air.
I have used brake clean to inspect for vacuum leaks and also done visual checks. Everything is good. Base gasket seems good. No change in RPM whatsoever when sprayed all around.
Questions:
Could the TPS be interfering or wrongly adjusted getting in the way of the throttle valves fully closing?
What lese could be? (throttle cable and cc don't seem to be though I could pull them off to be sure)
What am I missing?
What's my next move?
Vehicle: 1990 c1500 with a 350 tbi/700r4
Problem:
High idle at cold start (around 2000RPM) High idle when warmed up (around 900-1100rpm) Slightly rough idle when warm. Performance otherwise fine, no stumbling off idle or missing under load or revving in neutral. Drives great. Only a slightly rough idle that's too high and creates minor vibrations in the cabin when in gear at a stop light for instance. That's annoying.
Idles around 700-850rpm in gear.
As it does have a higher idle cold I would assume the CTS and IAC are working.
My guages all seem to read low. My voltage at the battery is 14.3v (new battery and alternator) but my gauge in dash reads 10-11v. Coolant temp on gauge reads 150 on average. Heater is pretty warm though. Seems ok. My oil, coolant and volt meters decrease in rythm with my blinker, I think this is a seperate cluster issue with grounds but worth mentioning.
History:
The PO tapped the idle adjustment screw and had obviously messed with it to compensate for this rough idle I'm assuming.
First thing I did was back the screw out a bit, this helped lower idle speed but the lower RPM did worsen the condition of the Idle. But I wanted less violonce when engaging trans.
So far I've:
I've changed the plugs, gapped at 0.35 which helped the idle slightly, but it's still not smooth. It has slight shudder every so often at idle when warm. Cold it's idling too high to shudder.
Pulled the cap and rotor and found crud on all the terminals inside the cap. Cleaned them all and sanded them lightly. Rotor too.
I've replaced the IAC with a used one I had lying around. This has changed nothing although the old one seemed to be pretty stiff (should you be able to push it in and out by hand?) and was very dirty. Trying to push the plunger back and forth it snapped. Should these be unmovable?
Either way, 'new' (known to be good used) IAC installed: No change.
I noticed when trying to reset idle speed accoriding to procedure (ALDL ports A-B jumped, ign. off, pull plug on IAC, start engine and adjust to 650rpm) that I can't get it too idle under 2000rpm with the IAC unplugged! That's the lowest, at that point the idle screw is fully backed out and the butterflies can't go back further. Also interestingly, the system is compensating. It adjusted to this lowest setting. Before the throttle was held open a little by the idle screw, now it's not touching and at first it died now it runs the same as before (described above) so it seems the IAC is doing it's job of metering idle air.
I have used brake clean to inspect for vacuum leaks and also done visual checks. Everything is good. Base gasket seems good. No change in RPM whatsoever when sprayed all around.
Questions:
Could the TPS be interfering or wrongly adjusted getting in the way of the throttle valves fully closing?
What lese could be? (throttle cable and cc don't seem to be though I could pull them off to be sure)
What am I missing?
What's my next move?
