89 7M-GE persistant, intermittent missing

[slugsride]

Too cold to work on it, time for more research. Thanks for looking, new member, first post. Have a 1989 7m-GE NA with 229K miles. All stock except K&N drop-in, head has never been off.

A few months ago it started missing on cold starts. It would clear up after driving a short distance and run fine until the next cold start. Injector cleaner and fresh gas did not help.
A few weeks ago when returning home after a 10 mile trip the missing got worse and did not stop. Miss is present now idling and driving, warm or cold, and at all RPMS.
There is no smoke, no knock, oil is clean. Missing is very intermittent, sometimes 4 or 5 in 10 seconds, sometimes it will run smooth several minutes before the next miss.

Items tested: TPS, IACV, MAF, fuel pressure, injector resistance, fuel pump relay and resistor, compression.
Items replaced: Coil (new), cap, rotor, distributor.
Checked: wires, plugs (both new 12k mi. ago)

Of the many forum posts I've read the ones that match these symptoms seem to point to broken ground or injector wires, but have not found any yet.
Any thoughts on what I have missed or where folks have found broken wires?
Thanks!

 

[hvyman]

Perform a block test and compression test.

 

[slugsride]

Checked compression on each cylinder when I checked the plugs and they were good. Need to check again with all the plugs out when I can leave it inside. (trying to keep a convertible in the garage during the crappy weather)
Will have to put a proper leakdown tester on my Christmas list!
What makes me lean away from BHG or rings is no other related symptoms, and when it is not missing it feels very normal (if only for a minute or so).

 

[hvyman]

What was the fuel pressure and injector resistances?

What brand and type of plugs did you put in?

 

[fox74]

An oil leak from valve cover had caused oil build up in the spark plug wells, this was the reason my car had intermittent missing.

 

[slugsride]

hvyman; fuel pressure 36 psi w/regulator, 43 psi w/o, injectors 13.6 to 13.8 ohms (normal for '89), plugs NGK BCPR5EP11

fox74; Was what I expected to find, and have had that problem before, but they were mostly clean this time (only 12K mi. since new).

Thank you both for the replies!

A couple weeks ago I was rechecking a few items, decided to start the car and the miss was no longer intermittent, I had a dead cylinder.
Pulled each plug wire and found #6 was dead. Pulled the plug, looked fine but was wet with gas. Checked compression, was good.
Checked wire resistance, ok. Checked wire with another plug, good spark. Could it simply be a bad plug?
Put a old plug in #6 and went for a test drive, no miss! Have made a few short trips since, no miss, runs normal.

At my age I should know better, but NEVER, ever trust a spark plug!

I guess what the next question is what killed a good plug at 12k mi.?

 

[SiberianDVM]

Just saw this thread and I am chasing a similar problem. Freshly rebuilt 7M-GE, ran fine during breakin. As soon as we got it to Rd Atlanta for a CCWS race, it developed a miss. We ran it flat out for 14 hours anyway, and I was expecting it to blow up any minute. When I got home and started the diagnostics, all I could find was a fouled #3 plug. They were new plugs but they were NGK Standards instead of Laser Platinums (brain fart). Compression test was normal (have leakdown tester on order). Replaced plugs with Laser Platinums and replaced plug wires even though they tested normal. Ran much better, but is still very slightly rough at idle. TPS did not test normal, and I got a code 41, so I ordered a new one.

So I'm waiting on parts. We had replaced all 6 injectors with green refurbs. If the leakdown is normal, I am going to try swapping the #3 injector, as the new plug is showing small deposits after only a few miles.

 

[SiberianDVM]

#3 fuel injector was bad, along with a not so good TPS. Replacement of both solved my mis-fire issue.

 

[jetjock]

Nice job and thanks for reporting back...