1999 saturn sc1 sohc. bought it last year. pretty clean car ran decent. did plugs, wires, ignition coils and the intake gasket (had the intake gasket leak). about 3 months later head gasket disintegrated. worst head gasket ive ever seen. had my friend help with the hg but we kinda messed it up so last month i finally got around to redoing it. car started right up amazingly but heres where the problem is:

ISSUES: starts up revs pretty high ~1500-2000k. as it warms up itll drop down to below 500 (from what i read online most likely the coolant sensor) but the other issue i ts having is when you rev it up the rpms will drop by themselves all the back to 500 even though im holding throttle steady. when you lift itll drop down to 150 and sometimes stall sometimes it saves itself.

i havent found any vacuum leaks, just replaced tps, cleaned throttle body and egr, put in new spark plugs and wires again, checked all the fuses, scanned for codes (there are non although the wrench light is on which i believe is a bulb out)

any advice would be great im tired of buying parts without it working.

 

wrench can be other things as well. Get a more highly capable scanner.

I've been thinking about this since you posted.
Cold startup should be to 1200 ish rpm, then down to maybe 650-750 rpm for an automatic.

The way I understand it, the PCM sets the desired idle and adjusts the hell out of everything to make it happen. 500rpm just sounds too low. IF it were the coolant sensor, it would prob be reading cold so I would expect the rpms to land on the high side of 650 at idle.

If you removed the throttle body before removing the head, the idle air control (IAC) may have gotten damaged, or the manual idle adjustment screw bumped. The only time it is advised to mess with the idle adjustment screw is if the throttle body is replaced. If you didn't get it back on just right you MAY need to adjust this. I'd test the IAC FIRST.

I'm not sure if a stuck open IAC would contribute that much to overall performance away from idle.

" bidirectional vacuum" related issue at hand at throttle body?

Andy?

 

I removed the tb to clean eberything after the hg when I noticed the low idle. I'm going to replace the ects when it arrives just to be safe. I removed the iacv and map sensor the other day and cleaned them now when it starts it goes straight to 3k (gonna check the harnesses tomorrow and see which of those sensors are the culprit)
what do u mean by iacv stuck open? Is it supposed to move? Mine doesn't move at all.
also I don't think I bumped the idle screw, or at least its not the main issue I'm concerned about since the rpms drop by them selves when I hold the throttle steady

 

Some cleaning solvents damage sensors. Even the throttle body specific cleaners have detailed instructions to not let the chemical touch the electronics. I only clean throttle plates carefully then i clean the idle air bypass with the engine running. I suspect the throttle position sensor got "cleaned". I have never "cleaned" a map sensor, they either work or they don't. More electronics are ruined by engine cleaning than anything else. It is kind of like flood damage a little at a time.

 

The map sensor is the only one I was really concerened about cleaning since it was pretty gunked up. I think I used electronic cleaner just to be safe. Iac I just sprayed mad cleaner on a rag and wiped off a little sludge.

 

The computer uses the iac to control idle speed. So it is a pulse width stepper motor that the computer uses to pinch off the idle air bypass to raise or lower idle.
Is the car driveable at all? Have you had all these problems only after the head was off?

 

Well, as usual I was a bit off...

Did you "clean" the new TPS before you tried starting the car 1st time after the HG work?
I would think you replaced it because of the messed up rev behavior, not proactively. Please indicate order of events.

Assuming new TPS did not get cleaned between install and engine starting, you may wish to re check pt num; or you may have gotten a brand new dud out of the box. Chances are higher if you got it from Autoblows or Advance.

Unless i am oversimplifying it, the TPS is just a resistor with a sliding tap whose position changes with throttle movement. It's a simple physical measurement, as the R (and therefore V drop) betw the tap and ground changes as throttle position changes. PCM reads V drop, translates into throttle position (V drop vs actual thrott position is premapped), throttle position determines how much and when fuel is provided and the timing is adjusted accordingly.

Given the above, it's a weird failure mode for a TPS - -especially if it is that reproducible. It'd be like the center tap sliding on the resistor and then losing contact, falling back to a constant resistance. I guess that's kinda consistent w what you see, but I would expect it to be erratic as hell if the TPS is damaged in this way. Maybe I don't have the physical model correct.

 

chronilogical order:
1) blew up hg
2) replaced hg, car started right up and ran fine but as it warmed up it started idling really low
3) cleaned tb, egr, made no difference. now that the car had done a few heat cycles i tried to rev it up a bit which is when i noticed the rpm drop
4) replaced tps (did not clean it), same issue just smoothed out idle a bit
5) went through to clean tb again, check lines and map sensor, this is when it revved up by itself. took map off again cleaned it again with electronic cleaner as well as the connections and it was back to the 1st problem of low idle
6) today i just replaced the ects and still no change but i think i found the problem. with the plastic line to from tb to charcoal purge valve unplugged it idles at ~600-650 when warm and will maintain throttle. with it plugged back in it goes back to low throttle and wont hold revs.

so im going to be replacing that next but until i do could i drive it with it just unplugged? or is there like a easy temperate way to 'delete' the egr/charcoal system?

 

Cap them both off and take it for a drive. The charcoal canister may be flooded with gas. I would repair the factory system.