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Followup To
Question -
What is the best way to narrow down which valve it would be? (The engine had a complete overhaul, including heads approx 35000 miles ago). Is it worthwhile trying to repeat the cleaning process?
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Followup To
Question - I tried the "Top Engine Cleaner" treatment. Unfortunately, this made no difference. Would the vacuum fluctuation be causing the rough idle? Or is the vacuum fluctuation a symptom of the rough idle? Any other thoughts?
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Question -
In the last couple of weeks the car has started to act up. When first started in the morning the engine backfires ? (popping heard in exhaust). Once the engine warms enough to idle down, the popping stops, but engine idles roughly. When driving the car it hesitates when driving at low speed(30 -40 mph). At higher speeds the engine does not hesitate. The car does not have a tach, but it seems that the problem only appears at lower rpm. This hesitation problem improves to the point where it almost dissapears once the car has fully warmed up.(car does not have a temp guage either, so I don't know at what temp it starts to behave better). I had installed new plugs aprox 1 year ago(Bosch Platinum) and new wires (Bosch) last week. I tried a different MAF sensor, IAC valve, EGR valve. I did a compression test, results 153 -168 psi. I checked the vacuum and found it to be approx 16 In Hg. I noticed that when the engine was idling roughly the vacuum would drop to 14 then back up to 16. Can you tell me what the vacuum should be and if the fluctuations indicate a vacuum leak? If it is a vacuum leak how does one try to locate it? Any other suggestions would be greatly appreciated.
Thanks
Answer -
Sounds like an exhaust valve sticking when cold.
Use some top engine cleaner, and change the oil.
The vacuum should be close to 18, but mostly, it should be steady, and not bouncing the needle around.
Van
Thanks for the information regarding "top engine cleaner". Is there a brand name for this? Do you know where it can be purchased?
Answer -
Purchase at any parts store.
STP, Barhdol, Marvel....ask the parts man. Tell them you want a cleaner, not an oil additive.
Van
Answer -
The vacuum fluctuation is probably because a valve isn't closing completely, and that is also causing the miss.
AnswerThe BEST way to tell which valve is sticking is to remove the spark plugs, and install an adapter, like a compression tester, with an air hose connector in it, and connecting air to it.
The firing order is needed, because you need to get whichever cylinder you are testing at the time, to a position where both valves for that cylinder are closed, and prefferably the piston is at the bottom of its stroke.
Then when the air is applied, listen to the intake, the exhaust pipe, and even inside the oil filler cap, to see where the air escapes.
If you have an exhaust valve sticking, you will hear more air flowing in the tailpipe on that cylinder.
This will require some expertise in knowing how far to turn the crankshaft from TDC, and for each cylinder how many degrees to turn it to get the valves closed, and the piston down.
The reason for wanting the piston down, is because the air will blow it down, and possibly when your fingers are near a belt etc.
Actually, just a compression test should tell you which cylinder is involved, and if you are hearing popping out the exhaust, I would bet it is an exhaust valve.
Van
