rompn4x

While sitting in a drive-thru yesterday I noticed a "thump thump thump" feeling and my truck was slightly surging when I let off the brakes. I shut the truck off and restarted it with the gas pedal floored so it would not start, just would keep cranking and I noticed the starter has a different sound now. Sounds almost like one cylinder has low compression I don't think I did anthing to hurt the engine. It literlly just appeared yesterday. I did have to gun it pretty hard yesterday on the freeway to pass a truck and sometimes I get detonation I hope I didn't damage anything. Engine is a 4.6 and has 184k miles well maintained. Ill go ahead and replace the plugs and wires but im not sure about the compression until I do a compression check. Should I continue to drive the truck? It doesn't feel like a totally dead cylinder but there is a noticable idle change but seems to smooth out at higher rpm's.

rompn4x

Or could this just be a bad coil?

rtg143

Go to AutoZone and get a free code check. That should pinpoint the cylinder. Then Pull the plug and check it and the compression. You can switch the coil with a different cylinder to see if the miss moves with it.

If the compression is VERY bad, cranking without starting it will have an uneven cranking. When the low cylinder is supposed to be on compression, it will turn faster at that point. We used to do that when a car was being traded in and didn't have time to do a complete compression test when the car had a miss.

EZ-HITCH

I am a novice mechanic at best. With the exception of oil, spark plug and brake changes; I usually fumble my way thru most repairs.

I have a 97 150 that has 275,000 miles, I just tried to check the compression. Seemed like a straight forward process. Is it possible that my truck could still be getting 90+ on the compression gauge? That just seems awfully good for a truck with that many miles on it. I have always run Mobil1 synthetic in it. I only checked the one cylinder, but I figured they should all be close to that shouldn't they??? I mean unless I have 1 good one and 7 bad ones.

I also noticed that if I cranked it longer it went up more, but the 90 was the first number it jumped to so I figured this was the compression from the first stroke and it went up slowly from there.

Am I doing something wrong? (besides only testing the one cylinder)

rtg143

Since the truck was well cared for, as your's was, they will have pretty good compression. As long as they're all close, within 10% of each other.

rompn4x

I must of had a stuck valve or something because the engine smoothed out and now it cranks over normal Maybe run some mystery oil thru it???