PCV / Vacuum / Idle Expert Needed!

[BackFromTheDead]

Ok, so here's the scenario:

* Oil on the outside of the PCV grommet and valve, and inside the valve a little too. I tried tightening the other end of the PCV pipe that goes into the intake, but it didn't seem to help.

* In-cab vacuum gauge perfect at about 19 when car first started (with a little choke to help). Idles a little rough at start, then smoothes out

* Drive down the block, stop at end of street, push choke in, vacuum nicely at ~19, idle smooth.

* Drive about a half mile, come to a stop at light, and it idles a little rough, and vacuum drops to 10-14

* Drives fine on main roads. After driving a while and coming to a stop light, the vacuum may be at 18-19, or it may be at 10-14 -- there's no rhyme or reason.

* Pull into store, turn off engine, come out and re-start - Idle nice and vacuum nice at 18-19.

* Drive some more, then at stop light, vacuum back to 10-14

Anyone have any clue what could be causing this??? Either my vacuum should be nice at idle or crappy at idle...but why back and forth?

 

[fliphkd778]

Hmm...if it's going back and forth and still running...
You could have a vacuum leak somewhere along the manifolds that's opening up as the truck warms up, or the ignition is changing a small bit to make your truck idle a little rough and throw off your vacuum. A while ago my truck was having trouble idling after a while on a main road, turned out that the distributor wasn't retarding ignition for coming down to idle at stoplights.
You might check the vacuum control valve(VCV) that controls the vacuum advance. It's located under the air cleaner, it's half white, half black grayish.
This is fsm page for checking the vcv.

 

[BackFromTheDead]

Thanks for the pic and the tip, Flip - I'll definitely check out the VCV as soon as I can. I'm hoping it's something like that, and not valves, etc. But I figure since it idles/vacuums perfect sometimes, and crappy others, it's gotta be something to do with the truck being cool or warm.

Two years til I can de-smog in NJ...

 

[BackFromTheDead]

I checked my VCV, and it does seem to be old and partially clogged. I'd normally just buy a new one for the hell of hit, but then I saw they are around $70 from SOR, and I can't find anywhere else.

Does anyone else think this could really be causing the weird low vacuum/rough idle problems I'm having that I describe above? Sometimes vacuum/idle is fine, sometimes the vacuum gets very low (~10) or even so low the engine stalls. There's no rhyme or reason to why it does it and when it does it.

I just don't want to drop $70 if that's not the problem...

 

[FJ40Jim]

Bret, to determine if the VCV is really the problem, just block it off temporarily. If symptom goes away, or becomes steady state, you know you're onto something. If it makes no difference, then it's probably not the problem.

FWIW, I would lean toward a manifold gasket leak...

 

[BackFromTheDead]

Bret, to determine if the VCV is really the problem, just block it off temporarily. If symptom goes away, or becomes steady state, you know you're onto something. If it makes no difference, then it's probably not the problem.

FWIW, I would lean toward a manifold gasket leak...

Thanks for your response, Jim. I tried to block off the VCV by clamping off or plugging all the lines that go into it, but this didn't seem to help at all.

In order to see if the VCV is the problem, should I be trying to simulate constant positive airflow, constant vacuum, or neither to the distributor cap?

I'm hoping it's not an exhaust gasket, but I have a feeling you're right, and that's what is causing it. It's just so strange that when I first start the truck when it's already warm, it idles perfect with a steady vacuum of 19. But then after driving a mile and coming to a stop, it the vacuum drops to ~10 or even lower, and sometimes stalls...

I would think that if it was a problem with the exhaust manifold or gaskets, the problem would be constant rather than intermittent.

Thanks again!

 

[FJ40Jim]

Vacuum numbers are misleading. Ignore them.
Focus on the symptom: Starts up and runs fine initially, then gets weak.

Sounds like the engine is running out of fuel. Could be dirt in the carb, weak fuel pump, dirt plugging the in tank strainer, plugged fuel filter....

 

[BackFromTheDead]

Vacuum numbers are misleading. Ignore them.
Focus on the symptom: Starts up and runs fine initially, then gets weak.

Sounds like the engine is running out of fuel. Could be dirt in the carb, weak fuel pump, dirt plugging the in tank strainer, plugged fuel filter....

I put a new fuel pump on about a year ago, and a new fuel filter about 3 months ago, so those should be OK. Also, it runs and accelerates fine when driving - the only problem is when idling at a stoplight, where it'll either begin to idle very roughly, or just plain die on me.

Maybe I'll re-check the timing and A/F mix...any luck on setting this with a CO meter, rather than a tach? I tend to get inaccurate results when doing it with the timing light tach, and can't quite figure out exactly where the max RPMs, etc are.

 

[FJ40Jim]

When it idles rough and dies, pop the hood and look at the carb sight glass. If the fuel is way high, that's the problem.

Timing is usually set/checked w/ a timing light.
A/F can be adjusted w/ a CO meter or a tach.

 

[BackFromTheDead]

Thanks for the tip, Jim...I'll chec the carb sight class.

Also, there's a new development on this: I noticed that if it's idling rough w/ low vac, and about to die, but I let it sputter and idle away without doing anything for a a few mins, the idle and vacuum will slowly return to perfect.

Then, if I drive another few miles, and come to another stop, it's idling rough again. Any idea what this could mean?