Persistent misfire

[92302expo]

So I’ve had a persistent misfire for the last week, so I replaced the plugs nothing changed and swapped the wires and coil pack with the ones from my parts truck nothing changed so I did a compression test 1=190 2=195 3=180 4=160 5=185 6=190 all of which is in spec. I then read the codes and had codes 629 and 566 which is unrelated as my truck has a five speed swap with an auto computer and code 116 so I changed the engine coolant temperature sensor with no change. So i cleaned the maf just cause and the misfire went away so I hopped in and road tested it and it was good for about three minutes and then it started missing again so I decided to try changing the maf with the one from the parts truck but it did not change so I decided to try changing the ignition control module but again no change. So I decided to change the fuel filter (almost 30,000 miles since last filter change) and I did a fuel pressure test before and after and it had 38 psi without the engine running and 40 psi with the vacuum line disconnected while the engine was running and it didn’t change with rpm for both tests. Can anybody think of anything else to check that might be my issue because I am scratching my head.

 

[410Fortune]

why is cylinder 4 at 160 psi? Within spec is one thing, but all cylinders should be within 10-15% of each other. With the results you get on 5 cyl the other one looks a little low.
You can add a little engine oil to #4 and repeat the compression test, does the number rise?
Do you ever notice a loss of coolant?
What does the spark plug look like on #4? compared to the other cylinders?

 

[MrQ]

410, for my own edification, I assume that adding engine oil to a cylinder temporarily seals the piston allowing the cylinder to read at a higher compression?

 

[92302expo]

When I check the coolant every couple of months its maybe a quart low and here’s what the plugs look like the oddball is cylinder number 3, and I have yet to add oil before a compression test.

 

[410Fortune]

@MrQ it is a good way to check/test compression issue around the piston and rings themselves.
If the numbers do not increase when you add the oil then you can assume it is not piston rings and you have a valve seal or gasket issue.

Quart of coolant is going somewhere..........
Cyl 3 spark plug is the dark one??
Cyl 4 had the low compression numbers??

whats that head gasket seal everyone around here loves? K seal or something like that?

You could tear the engine down to the lower intake and have a look see, but it might be time for head gaskets/heads
That quart of coolant is going somewhere.........

 

[92302expo]

Looks like one of the injectors died, so I swapped in the set from my parts truck and it runs great.

 

[MrQ]

There you go!

Glad it was a simple fix.

Still doesn't quite explain the lower compression on that cylinder, though.