rough idle in park???

[marcosv6]

Well, the majority wins. I reverted back to the 60s' and used the old "tube in the ear" trick. Sure enough, the intake manifold gasket is leaking in two spots on the right bank of cylinders. You can actually hear the hissing noise without the rubber tubing but I plan on shoving a tube in the Isuzu service manager's ear so that he will have no doubts. Hopefully that will cure the problem. Only have about 500 miles left on warranty.

Triathlete is correct about the misfiring. At low idle speed a vacuum leak is very noticeable. At higher revs, the amount of air thru the leak is small in relation to the total air intake.

Helps to have a 1970s' British sports car (1971 Marcos). They all leak something! But it's fast!

[johnnyapollo]

I learned the same tube trick tuning a pair of side-draft carbs...

-- John

[marcosv6]

Yep, works better on multiple carb set-ups than any synchronizer I've ever used. I've tuned Webers', Solexs', etc. with the tube. I wonder if it would help with my ex-wife?

Don

[autox-racer]

where on the intake is the leak? at the top split gasket? I found one small leak on the drivers side ground wire on the long intake bolt. There was air sucking past the head of the bolt because the ground wire had a slot in the eyelet. I moved it to the fuel rail bolt and it seems to have helped a lot.

I listened today a little bit with my stehoscope/hose and did not hear a lot of noise. But that was when it was cold. it seems my rough idle is only after warm up.. which leads to vacuum leak.. as things expand and rich warm up goes away. I also used ether and sprayed in the area of the leak and it almost killed the engine.

I changed my plugs to cheap bosch platinums and blocked of the egr with a plate.. no change. other than i have a code now i have to check since engine light came on.

more later.....

[marcosv6]

Fixed! Just got it back from the dealer. New intake manifold gasket installed. No more surging when put in park, and transmission operates smoother (?). Nice smooth idle.

Dealer covered under warranty (has 59782 miles). Thanks for the help.

Don

[Tobert]

Bah humbug. I noticed the hissing this evening in almost the exact same spot. I used the Carb Cleaner/Ether trick to approximate where it is and it's right by the EGR. Tightening the EGR did not help.

Guess I'll try the tube trick and order new gaskets tomorrow

Good thread - here are some keywords to help the search engine.

intake gasket leak hissing rough idle surging stall stalling vacuum gas mileage howl whistle