YAIT - Yet another idle thread...

[CharlieS]

Circling back again, this time for a mea culpa post. I found the massive vacuum leak, and it was indeed self inflicted.

While doing an inspection of the engine both visually and listening through a length of tubing, I found a potential source. There was the edge of a bracket clamped between the upper and lower plenum halves.

I loosened the bolts, removed the bracket, retorqued everything, and it is idling "like a sewing machine" now.

I am embarrassed that I caused this, and that it took me so long to diagnose, but I felt it was important to "fess up".

Thank you to everyone for the patient, friendly, and generous guidance, advice and tips.

 

[FMC80]

Circling back again, this time for a mea culpa post. I found the massive vacuum leak, and it was indeed self inflicted.

While doing an inspection of the engine both visually and listening through a length of tubing, I found a potential source. There was the edge of a bracket clamped between the upper and lower plenum halves.

View attachment 3464015

I loosened the bolts, removed the bracket, retorqued everything, and it is idling "like a sewing machine" now.

I am embarrassed that I caused this, and that it took me so long to diagnose, but I felt it was important to "fess up".

Thank you to everyone for the patient, friendly, and generous guidance, advice and tips.

Great post. Thanks for closing the loop on this. I have done way worse! Happy to know you found the likely culprit.

 

[mudgudgeon]

Circling back again, this time for a mea culpa post. I found the massive vacuum leak, and it was indeed self inflicted.

While doing an inspection of the engine both visually and listening through a length of tubing, I found a potential source. There was the edge of a bracket clamped between the upper and lower plenum halves.

View attachment 3464015

I loosened the bolts, removed the bracket, retorqued everything, and it is idling "like a sewing machine" now.

I am embarrassed that I caused this, and that it took me so long to diagnose, but I felt it was important to "fess up".

Thank you to everyone for the patient, friendly, and generous guidance, advice and tips.

Good job chasing it down

 

[CharlieS]

Adding a note here for future searchers. I think I had another vaccum leak - in the brake booster. Smoke testing didn't help find this one. The brake booster finally failed completely on trail last weekend. When the booster went out completely, I had horrific running at low idle speed. While troubleshooting, another 'mud member suggested capping off the plenum nipple for the brake booster hose. When I did that my idle got even better than it had been with all the other fixes. I replaced the booster and now the 80 is running great. I think I had a vacuum leak in the booster diaphragm. I'm sure there is probably a way to vacuum/pressure test the booster but I don't know what it is. I wouldn't rule it out as a source of vacuum leak if everything else checks out good though. This one lasted until 307K miles until catastrophic failure, and was probably slowly leaking for a good long while. Rubber eventually fails.