rough idle while in gear

Asked by May 25, 2016 at 03:52 PM about the 1994 Ford F-150 XL SB

Question type: Maintenance & Repair

I bought a 94 ford with the straight 6 (4.9L) apparently the guy had been chasing the check engine light that was on. it has all new plugs, plug wires, cap, and catalytic converter.  Now I found out the #5 cylinder only has about 50 LBS. of pressure while the other 5 cylinders have about 150 lbs. of pressure.  I am trying to keep from doing a head job on it because I just need to make it about 8 more months.  Any ideas or suggestions?

6 Answers

Also, would this cause the rough idle while in gear?

17,130

You've answered your own question. The pressure readings of all cylinders should not be equal. But they should be within 15 percent of the highest. Since your other five cylinders read close to 150 lbs, fifteen percent of that figure is 22 lbs. No cylinder should read lower than 150 minus 22 lbs, or 128 lbs. The cylinder showing 50 lbs is dead. Hence the bad idle and loss of power.

How long do you think it will run with a dead cylinder

17,130

Who knows. How long has it had a bad idle? I would not over- stress it Definitely no interstate driving at 70 mph. Take back roads or city streets to work may be your best bet. Check your oil regularly. Raw gasoline will play havoc with oil condition.

Thanks OJ. I'm not sure how long it's been going on. I bought it like that 2 weeks ago. I haven't nor will I do interstate speeds in it. Should I continue to run 10w-30 or change to a heavier weight? I also found out today that the EGR vacuum selinoid is bad.

17,130

Stick with the oil recommended by the manufacturer. Most engine damage from lack of oil begins at startup, when all the oil is sitting in the pan with the top of the engine dry. Using heavier oil just prolongs the time needed for the pump to circulate the oil to the top upon startup.

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