Mustang: getting to the root of the problem

here’s a rundown. i own a 1996 ford mustang base coupe with 75,700 miles on the engine. back on october 1st, i performed a mild tune-up, since i was hitting the 3,000 mile mark anyway, and used a engine oil flush detergent to clean out any gunk within my beast. little did i know that that kind of stuff you do not put into older engines, and roughly 2-3 days later, i began to get a rough idle and poor acceleration; loss of power.

after spending money on parts that i knew were in good shape, but mechanics i took advice from said otherwise and told me to replace them, i narrowed it down to a dead cylinder: cylinder #4 on my left bank, driver side.

a spark plug test showed cyl 1 and 4 were not firing and they were covered in oil, but 1 still held compression and evenly out among my other cylinders, they all held 150 psi, but 4 remained at 0 the entire time. i know the extent of the work but before i take the cylinder head off, i just need to be aware of what else could be the cause of no compression.

after i replaced the valve stem seals and kept getting the same results, another mechanic who later on did a compression test of his own told me im better off replacing the whole engine due to the fact that the rings are broken up, but i believe for a fact that this is primarily a burnt exhaust valve due to leaking oil from the valve stem seals. but i want to be sure so i wont waste time taking off the cylinder heads and the problem repeats itself.

do i look at the valves only or keeping going deeper into the engine?

During a cylinder leakdown test, #4 cylinder is placed at TDC (Top Dead Center), intake and exhaust valves closed. If the compressed air escapes back through the intake, the intake valve is leaking. If the compressed air escapes through the exhaust, the exhaust valve is leaking. If the compressed air escapes by the piston rings, into the engine crank case, that shows worn piston rings…

That is pretty low mileage to have that kind of problems. I suppose that, " with 75,700 miles on the engine." means that this engine was rebuilt. Before you dig into the engine, I think you should try to find a salvaged engine to install. Go to a Mustang forum and see what long blocks can go in so you can look for later years with fewer miles.

These engines have a history of bad head gaskets. Ford had a secret warrantee for these engines up to 100,000 miles but I think that it has expired. You may only need a head gasket, worth looking into. You really don’t know until teardown.

Junkyard engines are a crap shoot, especially if you order one over the internet. Its best to deal with a reputable local mechanic.

BTW, did you leave the “flush” in the engine? You are only supposed to use it for about 5 min then drain the oil.

well, i was also doing a coolant flush as well, so i ran it for 10 mins to get the engine to normal op temp.

Was the “compression test” done with an actual gauge screwed into the spark-plug hole or was it done electronically? Oil leaking down valve guides will not cause burned valves. Vacuum leaks will…but even badly burned valves will still provide SOME compression…How many miles on the CAR?

75,700, in my first post, but a analog gauge was used for the compression test, no electronics

Was this a chemical flush?

You said 75,300 on the ENGINE. I asked how many on the CAR…

" but i want to be sure so i wont waste time taking off the cylinder heads and the problem repeats itself. "

No matter what, the heads must come off, so you might as well get after it…

A “mild tune-up” because you were “hitting the 3,00 mile mark”?? What did this entail???

What was this engine flush?