I'll accept the fact that this problem may have no solution. This engine is way on the far side of past it's prime, but with valves adjusted, engine timed it's a tad sluggish off the line and the idle is somewhere in the 400
rpm range. The idle adjust screw on the throttle body is backed out to the max. If I pull off a vac hose on the common chamber the idle speeds up so I'm thinking there's too much gasoline for the air getting through. That would point to the engine temperature sensor. Is there a test for the sensor (short of swapping it out)?

I am not real in tune to these motors, yet, but it doesn't sound like by pulling a vaccuum hose would rule it to be too much fuel. If it was running rich at idle the fumes would be enough to most likely burn your eyes around the rear of the truck. When you pull a vaccuum line you are simply taking a load off of the engine, That is why it is idling up. However I am not sure what your problem may be. Someone will ring in that has had this problem before. Good luck

Helm has resistance specs, as temps go up resistance drops. I don't think its a real practical test unless you have an accurate reading of the coolant temps. also that sensor is not exactly the most accessable.

none the less, here's what it lists:

degrees F / K ohms
14 / 7-12
50 / 3-5
68 / 2-3
122 / 0.7-1
176 / 0.2-0.4

this seems like one of those situations that a # of things could contribute to your symptoms. I had similar issue that I never sorted out, and with my motor half apart I won't figure it out anytime soon. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />

off the top of my head, will idle speed kick up if you switch on AC/heater?

RT, have you adjusted both the mechanical idle stop and the idle bleed screw?

The former being at the front of the throttle body and acting upon the butterfly linkage, and the latter located on the firewall side of the TB.

The RS I picked up a few weeks ago wouldn't idle due to having compression issues (burnt valves). I adjusted the air bleed and it idled well enough to be driveable.

Can't recall for sure but I think you'd back out the screw to get more air thru.

You're definitely on the right track since popping a vacuum hose speeds up the engine. Obviously opening the throttle butterfly accomplishes similar and that's how most EFI motors are speed-controlled anyway.

All the IAC on a GM TBI-equipped vehicle is (for example), is a motor-operated valve which opens or closes to admit more or less air to adjust idle speed. I forget what type of IAC the 2.6 has but you may have issues with that.

It's possible you have other underlying problems but you may be able to at least compensate with the idle bleed.

BTW if your compression is dwindling due to the increased miles, it certainly can throw things out of whack. A compression test might be in order.

HTH & G'luck........ed

When you pull a vaccuum line you are simply taking a load off of the engine, That is why it is idling up.

Um, no. Vaccuum lines do not place a "load" on the engine. Pulling a line adds fresh air to the mix and the engine will speed up or idle badly, depending on the amount of fuel available.

For the above, it sounds like you a) misadjusted the idle speed (like Ed said) or the passage is clogged. If you crack the throttle a little bit, does it idle?

Michael

oops, sorry. I was just repeating what I was told by a mechanic years ago when I had similar problems on a old Chevy. Now I question the mechanic part of his title....Jury, disreguard that last statement...... <img src="/forums/images/graemlins/smile.gif" alt="" />

no stepper-motor type controller, rather more like an electric choke. this is another possible culprit, that dadburned 'auxiliary air valve' in Zu parlance:

I believe that was the source of my idle trouble - was stuck shut at start up, when it should be open to provide bypass for high idle. and if either of the large vac hoses to this thing are not seated right you will get poopy idle & running, as it is a fairly large unmetered vac leak.

just like the ECU temp sensor, it is buried almost beyond accessibility. <img src="/forums/images/graemlins/angry.gif" alt="" />

Thanks for posting the specs on that sensor, I've got a similar problem I'm t-shooting right now... Have heard of guys using those handheld infra-red thermometers for measuring coolant temps... point it a spot on the engine that is likely to be in close contact with the coolant, i.e. the thermostat housing... I'm gonna try it myself and see how the correlation is. Also wondering if you can access the wires for the sensor at some point in the wiring harness that doesn't require being a contortionist...

Anyone know if there is a practical diagnostic for the aux air valve ?

Also... I'm wondering if there is any reason that the aux air valve couldn't be re-located. The "other" end of the two hoses is very accessible, couldn't one simply cut the old hoses off and abandon the original valve, run two new hoses to the new valve, mount the new valve say on the firewall near the charcoal canister or some such, and extend the wire harness to reach the new valve ? Does it really _NEED_ to be where it is ?

edit: almost forgot... when you pull a vacuum line, won't the O2 sensor pick up on the change in the exhaust stream, and signal the ECM that something changed, which in turn will cause a mixture change ??? It's a "closed loop" right ???

Thanks,
Jeff

-let motor warm up to temp, shut off
-disco hose between air valve & TB
-plug hole in TB
-start motor again
-cover & uncover end of hose, if rpm changes more than ~50, valve is defective

resistance across aux valve terminals should be 38-42 ohms.

w/engine running there should be voltage at the harness connector, if not but wiring is ok then check fuel pump relay.



no reason at all, I've now got it set up on the plenum. if you want more details start a seperate thread. <img src="/forums/images/graemlins/cyclops.gif" alt="" />

Whew! Lots of stuff. So I was poking around over the weekend and did the old "push the EGR and the engine will stumble" only the engine didn't stumble. So I take it off figuring it's maybe clogged. It's not. I put it back on and start up. Still no reaction when it's pushed. Maybe the pipe is clogged? I undo the pipe with the engine running. Plenty of exhaust. Thread it back in and I thinks "maybe the internals on the common chamber are clogged". So I take off the EGR valve, with the engine running, and... no reaction at all. The engine continues to run.
I can feel the exhaust gas coming out of the upper port where the EGR is bolted up and I figured the lower port would be hissing from vacuum
(assuming it didn't stall out altogether) so I'm a little bit mystified at this turn of events. I shot the upper and lower ports full of carb cleaner with no change.

I recently replaced all the gaskets on the intake side (figuring that the low idle was because some busted gasket with a vacuum leak I hadn't found...it wasn't). I guess I'll have to get another set for the common chamber and take it off again. The only thing I can think is the gaskets
set I bought covered up some port that wasn't meant to be covered. So
presuming I get that working again I'll go down the list of suggestions and see if I can solve my low idle problem.