Rough idle when hot or under load

Shep, Thanks for the fast and detailed follow-up. It definitely is a misfire which gets worse the more the engine warms up. I observed it this morning, idling in the driveway, starting at dead cold up to full temp when the fans went on. Engine shake was progressively more pronounced that warmer it got. I took a vac reading from two unported sources with an old gauge I've had since Ford Y block days. It was a steady 15" until the A/C kicked on then dropped to about 14 1/2" and the shake was more pronounced. I had my wife sit in the car with her foot on the brake. No change so I don't believe it is a booster leak. Had her put it into drive and observed yet more engine shake and vac went to 14". The gauge may be tired now and not as responsive as it could be. The marks on the face of the gauge indicate 15" as late timing but I don't know how relevant that is to a fuel injected engine. The timing is on the button at 10 degrees in timing mode with RPMs under 800 as per spec. The "carbon deposits forming on the back of the intake valves" theory is interesting. Would symptoms of that be heat sensitive? Are there any products or procedures that you like to remove the carbon without taking out the oxygen sensors ?

Regards and TIA

On Sat, 6 Aug 2005 21:02:30 -0400, "Shep" wrote:

One BIG tip about that top end engine cleaner. DO NOT USE IT IN A GARAGE. Don't use it around your neighbors house if you want to avoid problems either. And DO NOT HAVE THE EXHAUST POINTING AT ANYTHING YOU WANT TO KEEP CLEAN. You won't believe how much carbon and crud will blow out of that pipe. And the smoke generated will make you think the world just exploded. I usually take the vehicle out into the woods or up on the state land (unused dirt roads mainly) and run the cleaner through it there. Still looks like a forest fire but it is out of the way. I buy the stuff by the case and run it through the fire departments gas rigs once a year and my own once every two-three years. Then change the oil and filter and do a tune up.

I know of a couple folks who didn't believe me when they decided to use some on a vehicle they had. They used it in a garage next to a nice convertible with a white paint job and top. Took a LONG time to get all the stink and crud off that car and out of the top. I went over and asked him why he didn't listen when I told him about what it does. He said " I didn't think you meant it! "

Again, many thanks. Because the vac is rock steady even though low, am I correct in assuming it is more likely the TB gasket or intake manifold rather than any one cylinder? I've done the power balance test as outlined in the FSM and no cylinder stood out as the culprit. I've sprayed around the TB with carb cleaner and also used an unlit propane torch to try to find any vacuum leak with no success. Can a TB have an internal vacuum leak? What are the most likely areas for a vacuum leak? I've replaced all of the small vac hoses on the engine. What has me puzzled is why the situation is the most pronounced when the engine is hottest. Am I correct in assuming this is because the fuel trim is then at its leanest and, any vac leak being constant, the net impact of such a leak is more noticeable?

Thanks for the heads-up on the procedure.

Anyone?

"Steve W." wrote in news:42f6a434$1_2@spool9- west.superfeed.net:

SeaFoam is a smilar product. about 5 bucks a can here...

"Steve W." wrote in news:42f6a434$1_2@spool9- west.superfeed.net:

Shit, heres the URL