This isn't on my truck but on Kyles truck. It's also a 98 with the 4.0l. But he's been having a really rough idle that shakes the whole truck for close to 4 months now. And he keeps throwing parts at it. It's got new spark plugs, wires, IAC, MAF, Intake Gasket, EGR and the downstream O2 Sensor. Now most of that stuff it was throwing codes for. But now it's not throwing any codes any more and the rough idle continues. So I hooked up my scan gauge to it and to mine to compare what the readings on everything were and the Fuel trim on his was way richer than mine.

My readings are:

Bank 1 Long Term - 8.95%
Bank 1 Sensor 1 - 0% - 5.78%
Bank 1 Short Term - 0% - 3.91%
Bank 2 Long Term - 12.5%
Bank 2 Sensor 1 - -3.12% - 0%
Bank 2 Short Term - -2.34% - 2.34%

His readings were:

Bank 1 Long Term - 0%
Bank 1 Sensor 1 - 29.61% - 38.12%
Bank 1 Short Term - 35.94%
Bank 2 Long Term - 0%
Bank 2 Sensor 1 - 32.01%
Bank 2 Short Term - 11.01% - 16.41%

Not really sure where to go from here with it. He's tired of putting money into it and now there's no codes to go off of anymore.

Edit: Forgot to mention that it idles fine when it's in open loop. It's when it goes into closed loop is when it starts to idle really bad.

 

when the engine is warm , remove the EGR vacuum line and check it for vacuum draw

the manifold vacuum to EGR valve switch may be stuck open from coolant deposits

 

In open loop(choke mode) the Computer doesn't use any feedback from the O2 sensors, it runs engine rich, with higher idle and advanced spark timing.

When engine temp gets up to about 160-180degF the computer will switch to Closed Loop and use the feedback from the UPSTREAM O2 sensors(sensor "1"'s) to adjust fuel/air mix.

Downstream O2 sensors(sensor "2"'s) are only used to test Cat converters, the data from these is compared to the data from the upstream sensors to verify cats are burning off hydrocarbons.

I would swap the new O2 sensor with an upstream sensor then redo the live data scan, and see if that bank is showing better numbers.

Yes, to above post, although a bad EGR vacuum switch would be bad full time not just when engine is warmed up.
But if there is an issue with EGR system adding exhaust gas to intake at idle it would explain the rough idle.
Computer doesn't use EGR system until it is in Closed Loop, so a problem in that system could only show up after warm up.

With the live scan what did the engine temp read?
Should be between 195-220degF(90-104C)

 

It was around 182 degF. Didn't really get any higher even after driving for 30 minutes with it being close to 90 degf outside.

 

Quick update found out it was a bad injector, but also did a compression test and drive side had low like 4 125 psi, 5 125, 6 110. 1 2 and 3 were at 130-140 psi witch is good for 220k on motor. Next step is new head for driver side and replace head gasket on passenger side. New lifters, push rods, rockers and injectors then it be good for another 220k.

 

I am at 300k on my 4.0l and last time I checked I was at 165 psi average.
test was:
Cold engine
all spark plugs removed
Throttle plate at full open(this also cuts off the fuel injectors)
Battery in good condition

But your numbers are all close to 10% of their average(128psi) so they are fine, the actual numbers are really meaningless since test parameters vary so much

$25 Vacuum gauge can tell you alot about engine condition, good read here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

For Chris98: someone has put in a 180degF thermostat, it should be changed to a 192degF thermostat.
People have some odd thoughts about thermostats, pretty much all of them are wrong, lol.
An engine running between 190-220degF has cleaner oil so better lubrication, that's from S.A.E. tests, condensation and blow-by contaminants are burned off better above 190degF.
Fuel economy is better above 190degF and below 220degF.

Running an engine cooler may sound like a good idea, but that's all it is, a sound, it ain't correct, lol.
A 180degF t-stat just shortens the life of an engine.

 

Still going to rebuild the top end I'd feel better in the end. Other reasons is the stock heads like to crack and sick of replacing the spark plug every few mouths because of it. Also I am getting a new thermostat, because I have to do a coolant flush with the rebuild.

 

Thought I would give a little update on this. This weekend we got her running good as new. There were so many things wrong with the truck that I am surprised it even ran at all. First off the timing chain had a 1/4" of play in it, the cam synchronizer had play and was noisy when pulled, #6 full injector was cracked, passenger head cylinder was cracked at the #1 cylinder and starting to crack at the #2 cylinder, driver side head the exhaust valve seats were gone on cylinder #5 and cylinder #6 and was cracked on cylinder #6.

So we installed brand new heads, new timing chain, new lifters, new push rods, new rocker arms, new fuel injectors, new cam synchronizer and all new seals.

Onto the pictures.

Overall picture of the driver side head. Notice the exhaust valves on cylinder 5 and cylinder 6.


Sorry shitty picture, but you can kind of see the crack between the valves on cylinder 6


Again sorry for the shitty picture, but you can really see the crack between the valves on cylinder 1.


One Pile of junk.


From this angle you can see how much higher the exhaust valves were sitting.


Passenger new head installed.


Picture of new head.



Both heads installed.


Starting to look like an engine again.

 

Thanks for the update

Good work