𝟏𝟗𝟗𝟐 𝐄𝐗𝐏𝐋𝐎𝐑𝐄𝐑 𝐕𝐄𝐑𝐘 𝐑𝐈𝐂𝐇 𝐋𝐎𝐖 𝐑𝐏𝐌𝐒

[jollykreiger]

Ever since i got my 1992 Explorer it has a weird "miss" at idle, almost like a dirty injector.
The weird thing to me is that the higher the rpm the better/ smoother it runs. It seems to be running on all cylinders and has decent power (I've had 3 4.0l ohv before this one) I did find the cat was fully blocking the exhaust so I fixed that.
At idle it smells rich and burns a ton of fuel too but on the highway I get about 15 mpg with mostly highway miles, a/c on and a Canoe strapped to the roof.

Ive pulled the plugs and found one cylinder to be running rich, swapped them out and it made no difference in the way it ran.
Replaced intake manifold gaskets (had a water leak) and fuel injectors since I thought that was the scource of the sputter.
I have not found any intake leaks other than the one i fixed the day after I bought it.
I had a spare coil pack so I swapped that out.

Nothing seems to change the way it runs though, any ideas here?

 

[jollykreiger]

Thought I would update with some new information.
It is definitely extremely rich at low rpms. New plugs come out dripping in fuel after running for about 10 seconds. Trying different things they always come out coated in fuel. All of them.

Every now and then the CEL comes on and goes out with a key cycle. When pulling the code it reads that its running rich but no other codes.

I swapped out the coil packs and ecu with used units and they have made no change.
Things I have checked
FPR, no leak on vacuum line, 35psi when connected at idle 45 psi when disconnected.
ECT, I dont recall the number but it did measure out correctly. CEL disconnected.
IAC, vastly changes rpm when disconnected and CEL, cleaned as well.
TPS, CEL when disconnected
MAF, .99vdc-1.01vdc when running at idle

I have even tried disconnecting every sensor (excluding crank sensor) 1 at a time to see if it would run any better bu no change.
Using borescope, watching valves. There is no discernible slack in timing chain.
Again with borescope on cylinder 1 now, moved to TDC and found it aligns exactly with 0° timing mark.

I am starting to wonder if the cam or crank key is half sheared and my camshaft timing is retarded a couple degrees or maybe it was installed a tooth off but who knows how long ago that could have been.

 

[Blmpkn]

O2 sensors?

 

[55trucker]

Get a scanner on the engine, being OBD1 the scanner display will show you what the realtime A/F ratio is. Stoich is 14.7, the engine running rich will display a much lower ratio value than 14.7.
Unlike an OBDII PCM/scanner that does NOT show what the actual A/F ratio is OBD1 does.

 

[jtaborsky]

Get a scanner on the engine, being OBD1 the scanner display will show you what the realtime A/F ratio is. Stoich is 14.7, the engine running rich will display a much lower ratio value than 14.7.
Unlike an OBDII PCM/scanner that does NOT show what the actual A/F ratio is OBD1 does.

How is this possible without a wideband sensor?

 

[55trucker]

How is this possible without a wideband sensor?

an OBD1 scanner doesn't display in the same manner as OBD2 does, with *1* the actual a/f ratio is displayed, stoich is 14.7, no wideband is necessary. The only area that I've made use of a wideband is in 80's-early 90's Chrysler turbo applications where megasquirt was introduced.

 

[Uncle Gump]

That old OTC is a gem...

 

[55trucker]

lol.....I've owned that for a little over 20 years now.