a bad head gasket? Recent tune-up. No CEL/MIL, no codes(including pending), IACV, TPS and MAF test good as far as I can tell. My symptoms include a high idle that lopes between 1500-2500rpm, sometimes sits at 3500rpm and sometimes drops to 500rpm(runs rough of course) and sometimes it sits at 750rpm like it is supposed to. I did look in the coolant reservoir and saw tiny bubbles in the coolant. Lots of condensation in the exhaust. No coolant leaks, no coolant in the oil, no oil in the coolant, no coolant on the exhaust(as a liquid and as a smell).

Any insight would be appreciated(as long as it pertains to my issue)

 

Just a few questions about the symptoms... Does the idle swing happen when the vehicle is moving, standing still in park or always? And does it happen when the engine is hot, cold or always? What is the weather like where you are now? Ambient temp? I'm guessing COLD...

I assume from your test of the IACV, MAF and TPS the ohm and volt values were all as specified but did you also test the continuity of the harness between the connectors back to the ECU and its plug? Did you also test all three temp senders (fuel, coolant and intake air)? Also, with that TPS was the resistance value decreasing at a perfectly steady rate as you very slowly opened the throttle from shut to full open (ignition on, engine off of course )?

If you have no visible or otherwise detectable coolant leaks, those tiny bubbles are probably just normal (pump cavitation).

 

Doesn't sound like a head gasket problem. First thing I would do would be to check for vacuum leaks and clean the IACV and throttle body especially where the IACV seats. The cruise control vacuum hoses get brittle and break off near the brake servo, of course you can tell by the CC not working.

 

You could have a vacuum leak, left over from a bad head gasket replacement. More then likely you have a dirty t/body for a dirty IAC plunger or carboned hole for the plunger in the manifold.

 

IACV, Throttle Body and MAF were already clean because I cleaned them for the po. I cleaned them again and made sure I got the IACV hole good with a q-tip. I did not check resistance across the wiring harness. Been too wet and cold for me. Generally, the temp has been in the mid 20s-low 40s(F of course). The idle changes every time I let off the accelerator and disengage the clutch or when I start the Disco. Example: I start the Disco in the morning, until I get moving, the idle travels between 1500-2500rpm. As I drive it, everytime I come to a stop sign or stop light, the idle does one of four things, either returns to normal(750rpm), drops to 500rpm, jumps to 3500rpm or hovers between 1500-2500rpm. It will also have a high idle while cruising(I can tell because when I let off the accelerator in any gear, the vehicle wants to continue accelerating(can't be good for fuel economy) I have found no vacuum leaks...all the hoses appear to be new and are still soft(not brittle and not squishy). I looked at the intake again last night before hitting the hay and did notice some RTV sealant(orange). I am thinking someone may have pulled the intake previously and instead of using gaskets, used rtv(cheap bastards). I could have a intake leak at the intake...

I will be ordering a gasket kit this weekend I guess, along with the radiator plug I broke(stupid plastic).
Thanks for the input. I'll check my wiring harness while I have the intake apart. That'll have to be next weekend cause the parts have to be shipped here.

 

Did you get a brass replacement radiator plug?

 

You could try the old reliable leak check.... spray starter fluid around the plenum base and any joint (even the downstream end of the MAF hose) in the air intake circuit.

That is if you can somehow get a steady idle (high or not) for the timeframe of the spray test.

I forgot to ask if you checked the values of the o2's... they naturally have a major impact on idle too.

 

You must have a leak, time to start checking, might even consider having a smoke test done to find it.

 

I thought about that, but was unsure of the thread pitch. Is it just 1/2" pipe thread?

 

ECT (engine coolant) should read about 700 ohms when warm at 185*

and finally only measure the upstream O2's voltage for now. They should read between .01 and 1.0 v with engine running and left & right should be about the same ... constantly fluctuating somewhere in the middle of those extreme values. Downstream sensors are just for comparison...

But you would have fault codes with that stuff ... Surprised there is no code or CEL lit? I do love puzzles.

I bet Mike is right and it is just a simple leak.