mightyjak

I bought a $400 1991 2WD 5-speed pickup with 280,000 miles from a buddy. It's run fairly well, but the thing lopes. The idle is pretty rough. I think the front cylinder rings are gone. I did a compression test today after a failed emissions test (way too much unburned fuel) and got around 60 psi on the front cylinder.

1. This was my first compression test. Pulled the plug, fit the compression tester and cranked it. Did I do this correctly? Seemed to idle better without the front plug. Tried to find specs for compression, anybody got them for 1991

2. Is there anything else that may cause a rough low end idle? Bad coil? O2 sensor? Can the O2 sensor be tested through harness or am I out $80 to replace it to see if it's bad?

3. If it is the rings, can I just pull the pistons and replace the rings? Or is this overkill for almost 300K miles? I'd rather avoid a full on rebuild.

The body and transmission are good and I'd love to keep it if I can fix it for cheap. It doesn't use oil. I have replaced plugs, wires, oil and filter but that has been the extent of my maintenance. The front plug was had a little caking/oil sludge. There are no dash lights on

Thanks for the help.

abecedarian

when doing a compression test you MUST remove all the sparkplugs AND hold the throttle WIDE OPEN. You don't want anything interfering with your readings.
After you do that, let us know the compression readings on ALL cylinders and how the spark plugs looked (pictures are even better)

mightyjak

I'm a little confused: pull all the plugs, crank it over using the battery while holding the throttle at wide open?

mightyjak

Shoulda waited a sec, found my answer on the Google. Handy, that.

Will redo compression test and post pics for your perusal. Thanks for putting up with silly questions.

Erik Beeman

Make sure the motor is to operating temps too, along with WIDE OPEN. You

mightyjak

Reran the compression test and these are the results:
1. 120 psi
2. 165 psi
3. 155 psi
4. 160 psi
Attached are the pics of the spark plugs:



Thoughts?

abecedarian

#1 shows some evidence of a rich mixture, or more likely oil contamination...
#2, 3, 4 all appear to be running somewhat lean... or slightly 'hot' (#3 moreso than the others)
given #1 seems rich, the O2 sensor would pick up on that and lean out the mixture, potentially causing 2,3,4 to look lean.
your compression results also seem to support some ring wear on #1.
I'm slightly troubled by cylinders 2-4 not having any deposits on the center electrode insulator....
#4 has some deposits on the ground electrode (more than #2 or #3).
your compression is really not 'that' bad on #1, but it seems something else is going on, else the other 3 cylinders wouldn't be as clean as they are.
may be valve adjustments are off on #1, but I don't see that fixing the problem. a rebuild is probably going to be the next step... unless the oil passage in front of #1 breeched the headgasket.
unfortunately, you won't be able to test that without a leak-down (and that probably won't tell either- you'll have to see the headgasket itself)

mightyjak

So, possibility of a bad head gasket?

Any chance that the slightly dirty #1 is enough to throw off the O2 sensor? In that maybe the O2 sensor needs to be replaced due to the dirty cylinder?

Basically wondering if there is a quick fix or am I destined for a rebuild. Is there a possibility of just re-ringing the the pistons without a teardown?

There was a guy with a shortblock w/pistons, etc. on craigslist. Thinking I could swap the rest of my running gear over to that....