I have a 99 Ford F350 with the V10 and 101,000 miles. I love this truck and engine until recently. It runs but is missing pretty bad. I just started out of the blue. I have had the coils and plugs replaced. The fuel pressure is spot on. My local Ford dealer did a relative compression test, told me that #5&6 cylinders were low and recommended a long block, at $6700 no less. I am pretty convinced this is not the problem as I can smell unburned fuel in the exhaust and my oil consumption has not changed at all. I am at a loss. Looking for any ideas.
You don’t believe that you have low compression on 5 and 6? Or you don’t believe that that could cause a problem?
The misfire should cause the CEL (check engine light) to come on. Does it?
I don’t believe this is the problem. They did a relative compression test and leaped directly to replace engine. No other diagnostic was used. There is no indication in any other system that I have a significant compression issue.
The CEL did not initially illuminate. This occured in the middle of nowhere Texas and I had to drive it about 200 miles in this condition. THe CEL did come on until the next day when I drove it to the dealer. Unburned fuel can be smelled from the exhaust at “idle” and the CEL could now be the result of fouling of the O2 sensor. I am at a loss as I don’t believe my local Ford dealer service center is credible and the next dealer is about 50 miles away.
If #5&6 compression is low enough to cause a misfire then you will get raw gas coming out since it’s not getting burned in #5 or #6 after all. Low compression without oil consumption tends to rule out the rings. Are #5 and #6 adjacent? Could just be a head gasket. Could also be one or more burned valves. Neither justifies engine replacement.
The coolant system shows no change and there is no visible exhaust, as in no vapor. As for burned valves, I don’t think that is the case as they would have to be virtualy no existent and even if that bad there should still be spark in a fairly explosive atmosphere. That is why I keep leaning to a problem with spark because fuel is getting into the cylinder and pumped out of the cylinder, just no ignition is taking place.
Compression checks can (and, probably often are) performed improperly. Good mechanics know of other ways to check for bad valves, excessive cylinder blow by, leaking intakes, internally leaking coolant systems, etc.
All of these test are done with smoke and mirrors. Wellllll, I exaggerate, they are done with simple tools: a vacuum test gauge, a cylinder leakdown tester, a cooling system pressure tester, and, yes, a compression gauge.
These are simple tools, but, a LOT can be learned with their use in knowledgeable hands. In unskilled hands, they yield very little information. Great tools don’t make great mechanics; rather, great mechanics make any tool great!
Save yourself a lot of time and money by finding a good (doesn’t even have to be great!) mechanic.