4.0 rough idle diagnostic help

[jballard81]

Afternoon! I've already read the tips here: https://www.therangerstation.com/forums/index.php?threads/before-performing-any-diagnostic-procedures-on-your-engine.82170/

-I replaced my fuel filter less than 1k miles ago
-same with spark plugs
-tested resistance on each plug wire (they were replaced ~2k miles ago) and they test in range
-tested secondary resistance on the coil pack, resistance was 10.55k, 10.95k and 10.59k (a little low, but probably fine??)
-confirmed nothing is living/clogging up air intake

I get an all clear from the KOEO test with a cheapo ODB1 reader.

I have a rough idle, shakes the truck. rpms seem steady at ~750.

I don't know the history of the truck. I've been doing a lot of neglected maintenance, pretty sure the odometer has rolled over twice and I'm in the low 200k range.
I have not done a compression test. I have not checked individual plugs for spark.
I was looking at picking up some reman fuel injectors but would appreciate some feedback on improving my diagnostic skills as my 'parts cannon' approach hasn't fixed it yet.

94 xlt 4wd

 

[bobbywalter]

start with a vacuum gauge, plug read, and the intake air tube condition and verify proper maf.

a compression test and leak down is always good for data.

 

[jballard81]

start with a vacuum gauge, plug read, and the intake air tube condition and verify proper maf.

a compression test and leak down is always good for data.

recently fixed my intake tube, i just got a vacuum gauge, will lookup how to use it in the manual or a youtube video.

will check the manual to verify the maf as well. thank you!

 

[bobbywalter]

minimal cost for the tools to do the leak down and vacuum....these are tools you should have anyway.


if some valves are leaky throwing parts at a rough idle can get expensive...and very irritating. better to know.

 

[jballard81]

Vacuum test results (i'm at ~300 ft elevation):
-18lbs at idle
-snap acceleration and vacuum dropped to zero (meaning no exhaust restriction)

leak down tester arrives tomorrow!

 

[bobbywalter]

thats good vacuum i would say.


but a air tube leak before the throttle body is sometimes a pia to figure out.

 

[jballard81]

thats good vacuum i would say.


but a air tube leak before the throttle body is sometimes a pia to figure out.

So my air tube was messed up pretty bad, I cut the TB end off entirely and used a turbo tube and combined it with TB end from a 97 air tube I pulled from the junkyard. Super glued and then coated with flex seal.

Of course it could still leak though. I also still need to test the MAF.

 

[bobbywalter]

what is with the hose looking thing next to the vacuum
reservoir

 

[jballard81]

what is with the hose looking thing next to the vacuum
reservoir

here?

that's a silicone turbo tube i found on amazon. I had to cut off ~3" of the original air tube b/c the only thing holding it together in the end was half a roll of aluminum heat tape lol. It was a tight fit, but i also super glued it then flex sealed.

 

[jballard81]

Plugs 2,4,5,6 all look like this which I believe is pretty good:

Plug #1 looks carbon fouled?

Plug #3 I think is ok, but might be getting oil introduced from a valve cover (threads are black)

Compression numbers:
#1 - 170
#2 - 170
#3 - 150
#4 - 160
#5 - 170
#6 - 170

***On #3 I might have had 2 threads in. It was very difficult to get the tester on there at all. I'm sure if I could have ensured a better seat it would probably be higher.***

I replaced my O2 sensors a month or so ago bc I was getting intermittent engine lights related to them. That might account for the carbon fouling on 1. Though, I read that if it's just one plug being fouled that it might be a valve or valve seal issue.

 

[jballard81]

Got a cylinder to TDC, connected the leak down tester and began applying air. Engine rotates at 60ish PSI..
So I set TDC again, set the brake, added wheel chocks, put it in drive and tried again. There must have been enough slack to allow it to rotate again

Even if that worked I would have to keep going back and forth setting TDC and then putting it back in drive etc.

Any tips would be greatly appreciated!

 

[bobbywalter]

you make sure both valves are closed and then i just hold the crank with a bar and put air to it enough just to make sure its not coming out of the valves too bad. i assume you have a differential gauge tester....so if its green its good. eieio from there.

 

[jballard81]

got a ratchet on the crankshaft and was able to hold it until the tester said it was in the moderate/good range and (then the ratchet would slip). Combined with what looks like good compression numbers that speaks to the valves sealing OK at least right?

It's tough for someone that's inexperienced to get that leak-down test done. At least by myself it was. Maybe i can get one of my kids to help this weekend. I only did a couple cylinders as well...

 

[JJMaine]

Have you cleaned or inspected the IAC valve?

 

[jballard81]

Have you cleaned or inspected the IAC valve?

I have, however I recall it being pretty nasty. I feel like it cleaned up well though. I need to check the service manual to see if there's a test I can perform with a multimeter.