92 sporadic misfire while engine hot and over 2k RPM

[PugetPounder]

I recently bought this 1992 4.0l w/ 160k for very cheap. Seller said it needed new plugs and I assumed that’s what was causing the misfire. I haven’t done a compression check or fuel pressure tesr(I have both testers coming tomorrow), but have checked the ignition coil, replaced the fuel filter, new plugs, new wires. There is no CEL illuminated but I’ve checked for codes, and have received
-Purge canister solenoid fault(haven’t seen this since replacing solenoid)
-vehicle speed sensor fault
-fuel pump secondary circuit fault
-O2 sensor fault/lean

Am I on the right track with compression and fuel testing? Has anyone experienced a similar problem?
Thanks in advance!

 

[Uncle Gump]

On a new to you vehicle with a drivability issue I personally feel a compression test is warranted. It will confirm or eliminate any mechanical issues with the engine.

Now always when I have a vehicle with a drivability issue... fuel pressure is typically the first thing I do. Because without the correct fuel pressure... the fuel system will never be right.

I should also ask... what plugs did you put in it? What plugs were in it when you bought it? I'm just a firm believer in motorcraft plugs in my Ford vehicles. I have fixed similar issues by simply putting motorcraft plugs in.

 

[Bear in the woods]

Vacuum lines can cause many problems also on new to you old vehicles
Broke one on my 90 2.9. Could barely
pull a hill. But would ruff idle and never die

 

[PugetPounder]

On a new to you vehicle with a drivability issue I personally feel a compression test is warranted. It will confirm or eliminate any mechanical issues with the engine.

Now always when I have a vehicle with a drivability issue... fuel pressure is typically the first thing I do. Because without the correct fuel pressure... the fuel system will never be right.

I should also ask... what plugs did you put in it? What plugs were in it when you bought it? I'm just a firm believer in motorcraft plugs in my Ford vehicles. I have fixed similar issues by simply putting motorcraft plugs in.

Thank you for all the info.

Glad I’m on the right track at least.
I did put in NGK double platinums not motorcraft plugs. And there were AC delco plats in when I bought it.

I forgot to mention the current gas mileage is about half of what it should be.

I’ll run both tests tomorrow and see what I get back.

 

[PugetPounder]

Vacuum lines can cause many problems also on new to you old vehicles
Broke one on my 90 2.9. Could barely
pull a hill. But would ruff idle and never die

I haven’t done a through vacuum leak check. It does run and drive, the power loss of the misfire is only for a split second and it’s very intermittent.

 

[Cliff_m]

Connect a vacuum gauge to the intake. Check for vacuum at idle and and off idle. The needle movement will tell the story immediately. This test will sort mechanical from fuel or ignition issues.

You can run through the ignition issues using a timing light on each plug wire. Watch for irregular or dim flashing.

 

[PugetPounder]

So following up original post.

compression, fuel pressure, and vacuum were all good. I swapped the Ngk plugs for motorcraft and noticed a little difference but still much of the same problem.
I stumbled onto another thread where they were having the same problem in a 94’ B Series and it ended up being the oxygen sensor. I guess when the engine goes from closed to open loop the real time data is not accurate causing the issues.

So I unplugged the oxygen sensor, got the engine warmed up and the misfire, hesitation went away. Just ordered a new sensor from rock auto, I will follow up again when it arrives and is installed.
Thanks again for all the help and ideas everyone!

 

[Cliff_m]

Keep us posted.
What you had want a misfire. Just low power from a lean condition if your diagnosis is correct. Glad you got something going in the right direction.