Ben Feral Selinger

I think I've got an 01 or 02 6.0L LQ4 (in a YJ truggy). When first installed, it screamed. Had no problem peeling out on my 42's. At some point, there was a very rapid loss of power. 0-60 was initially around 6.5 seconds, and is now closer to 11s. I haven't been able to really find anything overtly wrong though. Compression is a bit low at around 110psi +/- 9psi. I've got what sounds like a bit of lifter tick, and I have no idea what my oil pressure is as for some reason, every gauge I've tried, gives me a reading about 10psi off from the next gauge, but average is around 30psi.
Right at the bottom end off the line, it feels like I've got about half the power I expect, and there is no additional power made after about 45-50% throttle.
I've got heaps of logging, but I've never found someone who can send me a log of a known good engine, so I've nothing to compare to. I strongly suspect I'm pulling way too much timing on throttle, but everyone I've given the numbers to, gives me a completely different answer (it's either normal or it's pulling way too much, or not enough...). Trims look good, fuel pressure is stable, but I notice I'll pull all the way down to 8* on high torque, which seems ... not good. Again, lots of guys tell me this is normal/good, while other say something is horribly wrong and that it should be above 30* at WOT.

So at any rate, I cracked my oil pan the other day so I'm replacing it with an LS3 pan. While the pan's off, I figured it might make sense to have a look at the rod/main bearings. I've been told I can just swap the bearings in place without pulling the motor, and then of course I've also been told I can't . Figure rod bearings are cheap, mains aren't expensive, but I hate throwing parts at a problem without knowing the underlying cause. The only reason to suspect low oil pressure is the lifter tick. The motor ran dead smooth and no tick when I first swapped it in. The power loss occurred when the ticking started. Looking down the valve cover, I can see there's barely any oil being pumped up over the rockers. This again, half the guys tell me this is normal, while the other half say it's starved. I check two other motors, both running perfect, one is SHOOTING oil out the rockers and the other I can barely see any oil at all. Same problem with timing chain. Looks loose to me, some guys say it's way too loose, other guys say it's perfect. I've never found any actual spec on oil delivery to the head, timing chain slack, etc.

Also, when first installed, I was making an easy 20+mpg. I'm now getting 12 at most. Again, I can't find anything wrong. No codes, Everything seems mechanically fine. Trans is not slipping (according to OBD calculated gear ratio).

Anyone have any input before I just slap the new pan on and wait for my new motor to arrive in the spring?

Buzzmanb12

iTrader: (11)

Just a guess here... The big loss of MPG and performance suggest that the Front 02 Sensor/Sensors might be bad, but no fault code says it/they aren't the problem.

Ben Feral Selinger

That's my issue. I'm not entirely dependent on the ECM throwing a code to know what's wrong, but when the actual raw data also looks good, it's hard to suspect anything.
The only useful hint I've got is the timing getting pulled so low on hard throttle and WOT, without any knock being reported as the cause. I've been asking around for years about what all can pull timing without being reported, but every answer I've got so far has either been wrong, a guess, or ... wrong.. :p.

The lifter tick suggests to me that the oil pressure, at least to the top end, is weak, and maybe the lifters aren't pumping up enough and my valves thus, aren't fully opening (or possibly even closing), but I can't think of a way to test that either as a leakdown test will only tell me if they close properly. I have no idea how to tell if the valves are fully opening, but the massive loss of power and fuel economy really FEELS like a valvetrain issue.

All plugs look perfect after a few thousand miles (though I did have one plug crack, which caused all sorts of issues, and i was all excited when I found it, thinking it could answer all the problems, but it didn't).
Just can't find anything definitive wrong. Had heaps of other guys look at it as well, but no one's found anything to point a finger at.

bammax

Burnouts with big tires and now a lack of power and mileage makes me instantly think you smoked the trans. Every time I've burned up a 4l60 it's initially showed up as poor mileage and weak power off the line. Usually followed by a delay and loud clunk sound shifting into reverse.

Ben Feral Selinger

4l80e, and I rebuilt this one myself. Trans is not slipping. Calculated gear ratio matches as it should. I'm not opposed to the trans being the problem, but I can't find any indication to support this.
First trans blew up... because the extension housing broke off .
rebuilt this one myself as I had to crack it open to install the rear reluctor.
It's my understanding the calculated gear ratio would be a little math based on the engine RPM, input speed vs output speed. It'll definitely slip the ratio a touch DURING the shift, but once shift if complete, the ratio is rock solid.

SoFla01SSLookinstok

As you stated the ticking & power loss seemed to come at same time. Maybe the ticking is being read as knock & it's pulling all your timing?

Ben Feral Selinger

I'm not seeing much in the way of knock retard, at least not often enough to account for the loss at low RPM. Its' seriously about half the low end I had when I first did the swap. ECM will occasionally report a little knock retard of 4-5*, but even when it's not seeing any knock, it's still very weak. Essentially I'm expecting it to peel out with 40% throttle (like it did originally), but 40% now feels like 20%, and 80% is no more power than 40%. 40% seems to be the end of power gains, and it's about half what it should be .

cookseyb

Limp Home Mode??

Che70velle

iTrader: (6)

thunderstruck507

iTrader: (18)

If it has a MAF, start there. If it's off but still reading it likely won't show a code but the whole tune will be thrown off. Same goes for engine temp sensors and IAT sensor.

Next would be fuel pressure. Could have a clogged filter or a dying pump?

Some of the truck tunes have garbage that pulls timing on a WOT hit then feeds it back in. "Burst knock" is the term I believe...it pulls timing on the hit just in case the engine might have a chance of getting KR. It should be turned off if the vehicle has a tune IMO. But if it was running right before and now it isn't then that's likely not the issue.

Front O2 sensors can cause similar issues but your fuel trims should reflect a problem if that were the case.

Ben Feral Selinger

I always suspect the MAF, but don't know what to look for, mainly because I don't know what it SHOULD be reading. I've got lots of logs, and the MAF seems to respond to throttle and MAP readings, but I dont' know what the CORRECT MAF reading should be for a given throttle/MAP/load condition might be.

I did swap the MAF at some point, and that point may have been around the time I lost power. I swapped it because my trims were wildly positive, even though there were no intake or exhaust leaks, O2's seemed to be reporting correctly, and my fueling tables were all stock for the engine. I figured maybe I'd got a bad MAF. I tried tuning the MAF table based on O2/Trims, and got the trims stable in the single digits, but the truck ran like absolute ***** and made zero power. IE: not safe to drive on the highway. That was my first hint, but i couldn't ever figure it out. So I swapped the MAF and put my MAF tables back to stock, and my trims went to single digit.

The only other hint here is that if I disconnect the MAF entirely and try to run on VE, the truck runs horribly. In VE mode, it runs just as bad as it did when I tuned the MAF table. Without the MAF running, the truck barely runs.

thunderstruck507

iTrader: (18)

Does it have the truck style MAF with the integrated IAT sensor or does it have a separate sensor?

In VE mode it should be fueling based on: IAT, ECT, and MAP.

Ben Feral Selinger

Yes, MAF with IAT built in. IAT is working. Intake temp is correctly reported.
I thought with VE, it was assumed the MAF is missing, so it couldn't use IAT. How does VE work in .... most LS's where the IAT is in the MAF ?
I've seen loads of trucks run just fine with the MAF disconnected. Just a slight decrease in fuel economy usually.

The other thing I can note is that with the MAF plugged in, but the O2 sensors disconnected, it runs exactly the same as iwth the MAF unplugged, but O2 sensors plugged in.
Just runs like absolute garbage.

With everything plugged in and seemingly working, it runs fine, but just doesn't make the power I expect, and horrible fuel economy (10-12mpg right now, down from nearly 20 when I first did the swap).

thunderstruck507

iTrader: (18)

I've known them to run like garbage with the MAF unplugged due to the lack of IAT. Usually they run better than with a bad MAF plugged in but still not very good at all because the engine defaults to full negative and dumps fuel to compensate for what it thinks is colder/denser air.

It's my understanding Speed Density/ VE does need the IAT based on my research converting my car to speed density. My car uses a LS1 car harness and I'm using an adapter harness to connect to an 85mm truck MAF, so on mine I can just plug the IAT back into the factory harness when messing around with no MAF.

To me diagnosing a bad MAF can be tricky because comparing readings between logs is difficult as far as simulating similar conditions...it doesn't take more than a couple lbs/min to make things really bad. I always end up pulling a known good MAF from my truck to diagnose suspected faulty ones.

Engine coolant sensor is also crucial. Some vehicles have 2: 1 for gauge readings and 1 for the PCM only. Make sure the PCM is seeing accurate coolant temps.

This vehicle I assume has a drive by cable throttle body and not DBW correct? I've seen trucks with bad throttle body motors run like **** how you describe as well.

Ben Feral Selinger

Well on the MAF not being plugged in, I've driven other trucks where the IAT was definitely in the MAF and when we unplugged the MAF, it still ran fine on VE alone, so I dunno what's up there.
I'm actually on DBW.
That said, every bit of data from the ECU LOOKS good. I don't have any real physical gauges, so all my diagnostics is based on what the ECM reports. Coolant temp, air temp, air flow, map, o2, trims, ... everything LOOKS good to me. I don't know what a good MAP reading should be under various conditions, but I seem to get pretty similar looking numbers regardless of which MAF I use, except if I use the one I started with, which resulted in massively positive trims.

My understanding is that the trims are the final result, based on real O2 data though, so I've always assumed I can trust that if the ECU is trimming positive, it is because doing a calculation between MAF input and O2 input, the ECU found that we were not getting enough fuel for the amount of air we thought we'd put in. But I suppose that doesn't make sense, cause then the O2's would show rich immediately.
I wonder if trims are the difference between the MAF input + stoich, minus the O2? If that was the case, that would explain why one MAF gave me massive rich trims, but the O2's said everything was normal. Perhaps that MAF was underreporting, so we were getting more air than we though, thus running lean, so the O2's noticed this and the ECU dumped fuel to compensate?

Still, you'd think then that i'd run like ****, but that was the one time it ran awesome (unless the MAF was disconnected).

RockinWs6

I'd start looking why compression is so low in all the cylinders, maybe the timing chain has jumped a tooth.

Ben Feral Selinger

****. Can the chain really jump a tooth and still run fine, but just make little power, and not throw any codes?

thunderstruck507

iTrader: (18)

If it's DBW that would be my next primary suspect.

I'm not sure how people test them or if you can visually watch to see if it's getting WOT, it's another thing I had a spare of so I would typically swap out and look for change.

Ben Feral Selinger

I suppose there's that. I get WOT when I'm just sitting with the truck off, but I don't recall if I even checked my reported TPS at WOT during any of my logging.

bammax

If the chain jumps 1 tooth it's basically the equivalent of turning the distributor on an old school engine or installing the timing gear advanced or retarded as opposed to straight up. I'd imagine that the pcm would detect it from the cam and crank sensors, but I'm not sure if there's an actual code associated with it.