Rough idle 5.0

[rs2k]

I'm trying to track down the cause of an occasional rough idle and once in a while slight stumble upon acceleration from a dead stop.

The idle seems it's best after the idle RPM comes down from a cold start. It seems its worst after a hot start. I've also noticed the engine takes a little longer to turn over during a hot start than a cold start.

I have a rock solid 17 inHg of vacuum at idle.
I've cleaned the MAF a few times with MAF cleaner.
I have new O2 sensors.
I have new plugs, (recommended ones), new wires (recommended ones) and a new Camshaft Position sensor installed correctly. Cylinder one at TDC with the proper alignment tool used.
Engine has 130K miles and seems ot make good power. I have no problem towing a 4500 lb boat.

Any ideas on what I should do? Maybe my MAF is going bad? Maybe 17 inHG is too low and I have a vacuum leak somewhere? I suspect I might have a small leak somewhere because my fuel trim is about 20% extra rich at idle according to my OBDII scanner.

Thanks for any help!

 

[Turdle]

I think you are on the right track, if I recall a stock v8 should pull about 20-21


My 347 x cammed Mounty pulled 17 in at idle. Just for reference, I'll hook up a vacuum gauge to my truck and check right quick.

 

[rs2k]

I have a set of intake gaskets ready to go, but I'd prefer to find the problem instead of throwing money and time at it.

 

[Turdle]

warm idle vacuum on our 2000

Places to check for vacuum leak would be the brake booster-and the one way valve. listen for a hiss as you fiddle with it. The seal on the booster will sometimes leak.

Pcv valve and grommet --poor seal in lower intake
vacuum tree for FPR, EGR, HVAC

 

[rs2k]

Thanks for checking that!

I checked for a vacuum leak using the WD40 method. I didn't get a any change in RPM until I purposely put WD40 into an open vacuum line. I guess a 3 inHG drop wouldn't really be much of a leak though.

 

[Turdle]

OK, for a "Real Good!" vacuum leak check:

Take off the fan. It should not hurt to idle the engine for a few minutes without it.

Now,with engine running, and, using an non lighted propane torch, open the flow of propane and work the torch nozzle into the suspect areas, watch your vacuum gauge, idle speed.

Using a spray method of any kind with all the airflow of the fan seems ineffective, to me anyway. Good luck!

 

[rs2k]

OK, here's what I just did:

Removed PCV elbow hose from intake manifold and plugged both ends. Removed and plugged brake booster hose. Removed and plugged evac line. Removed and plugged red vacuum line and nipple from manifold/tee on passenger rear of engine (can't really see back there). Checked vacuum again: Solid 17 inHG when warm. (18 inHG when cold).

I'm really thinking if I have a vacuum leak than it's the lower intake. I need to get some propane.

 

[rs2k]

i just decided to do it. got the top intake off and found the egr valve had no gasket. inside of the intake is also a blackened mess. I'll clean it up, replace the gaskets and add some aircraft sealant as well as clean the fuel injectors while I'm in there and see what happens.

 

[Turdle]

So, how did this go?

 

[rs2k]

I had a bolt break while removing the lower intake. Front Driver's side intake bolt. It's seized pretty bad, I think I'm going to have to drill the whole thing out and helicoil it. Once I get it out I'll put everything back together and report on the outcome.

 

[rs2k]

Got the bolt out by welding a nut to the tip of it. It was stuck pretty good, but I think the PB Blaster did the trick. No way an extractor would have gotten it out though.

I'll put her back together after work.

 

[Spartan]

Please let us know the outcome, I've been fighting the idle issue for a long time now.

 

[rs2k]

Will do! I'm about to get back to work on it. Might have it back together tonight if I don't get too tired. I'm not really in a big hurry though.

 

[rs2k]

I soaked the intake manifolds and throttle and throttle body in degreaser for about an hour. Then I sprayed it down with carb cleaner. Then I sprayed it down with Dawn and a pressure washer. Letting them dry out now. The idle air valve was pitch black with carbon. The inside of the engine valley looks amazingly clean for an engine of over 100K (Looks closer to 10K), why in the world is the intake so unbelievably dirty with thick carbon. Is this all from the EGR? Maybe a bad PCV valve that stayed on too long?

 

[Spartan]

I'm guessing a combination of the two. Both can allow oil/soot into the intake.

 

[rs2k]

That's a big engine in a little space! I can't close my hands very quickly, I can't feel my toes, and my scratch covered hands are about 10% bigger than normal at the moment. I've got the intake back on and will have to get it finished up tomorrow... assuming I can still move.


Getting the lower intake onto the heads and block without getting the rubber oil seals and intake gaskets out of whack was a nightmare. I'm pretty sure they went on straight the last attempt, but it would sure suck if it didn't.

 

[Turdle]

Did you use the end seals or run a bead of permatex?

Normally for the front and rear seal of the lower intake the end gaskets are thrown away and a 3/8" bead of mega black ( not the normal black) rtv permatex stuff is used.

I never understood why fel pro would include the little gaskets but say not to use them in the instructions.

 

[rs2k]

I used the end seals and Permatex no 3 Aviation form-a-gasket. I've had great luck with it in the past.

Using the mega black rtv would have been a better idea.

 

[imp]

After all this bally-hoo, are your injectors original? Have you considered them as a potential cause of the problem(s)? imp

 

[rs2k]

I ran carb cleaner through all the injectors under pressure. They seemed fine.