Hey guys, this is a bit of a cross post from the tuning section, but after trying their suggestions, looking at logs for a couple days, and playing with my tune I think it might be an internal problem. The confusing thing is how intermittent it seems to be. I just have an LQ4 with a mild cam and 10:1 compression pistons, stock valvetrain. I started my truck the other day and it ran fine, stopped at the gas station and when I started it back up, it shook like crazy at idle and bucked when cruising at low throttle and threw a maf and random misfire code. I went home and checked my fuel trims and they were lean pretty much everywhere by 5-10 percent, so I switched to my speed density tune (which I had the fuel trims just about dialed in on last time I ran it) thinking my maf was just on the way out, after all its 200,000 miles and 16 years old. To my surprise, the fuel trims were still showing lean so I check my fuel pressure which seemed pretty in spec I think it was 55 key on, 60 engine idling and revving. I drive it around and get my fuel trims where I'm comfortable, but now it bucks just about constantly and my bank 1 and bank 2 fuel trims are generally nowhere near eachother 10+ apart in many cases so I give up for the night. I try again this morning and all is fine for a while, still bucking a little at light throttle cruise and pretty gutless until I hit about 2,800 RPM, then it all smooths out. after about 20 minutes though its bucking so bad its almost undriveable and the fuel trims were all super lean again and the left and right bank O2s were giving pretty different readings. Sorry for the essay, I was trying to be as descriptive as I could. I appreciate any pointers you guys might have, I'm at a loss here. I'm thinking it might be something with the heads, but the fact that it runs better at higher rpms and it seems to more or less come and go make me think not, and I'd really like to go through all of my options before I pull the heads. I'll upload my last scan in case anyone wants to look at it

 

I guess my question is, does this sound like something that could possibly be caused by valvetrain issues or anything else on the engine for that matter? Or does it sound like I should do more looking into my tune? I'm not great at diagnosing engines and have zero idea where to even start on this issue, I checked for a vacuum leak and couldn't find one (not that it should matter that much on a speed density tune from what I've heard)

 

May sound crazy but have you ruled out any electrical issues such as alternator etc? Bad ground somewhere? Think simple first.

 

I agree, look at simple things first before completely ripping stuff apart only to find nothing obvious


Did you check the purge valve for vacuum leaks with the pipe off? Super easy and takes a minute to check

 

Sounds to me like the oxygen sensors are bad - reading false lean and making the ECU trim up, which is making the truck run rich and causing the bucking.

 

Are you sure you don't have this backwards? Fuel pressure is normally lower at idle (under vacuum) and higher with just the key on.

 

A BIG +1 on this

 

I haven't ruled out electrical issues yet, I guess that'll be next on the list before I start digging deeper. It's funny, everyone always tells me I should start simple but I always jump right to the complicated answer haha. I didn't even think to check the purge valve for leaks yet, I'll have to try that out too. I did consider the oxygen sensors as a culprit, but I was hesitant to just replace them since just the two upstream sensors are almost $100 a pop. I wish I could find a cheap LSU 4.9 sensor locally, as that would tell me for sure. But they have been making some odd waveforms lately, so I guess its probably a safe bet that at least one is on the way out. Thanks guys! i guess I'll pick up a set of O2 sensors tomorrow and see what happens

 

Check for vacuum and exhaust leaks also.

 

bad load of fuel? ive had it happen...

 

I checked for vacuum leaks and wasn't able to find any, although I'm sure there's a couple spots I missed. I don't hear or see any exhaust leaks, but that doesn't mean there isn't one. I'm sure there's a more scientific was to check, but those are the only two ways I know of. I had thought at first it was a bad tank of fuel, but I got more a few days later at a different station and that didn't seem to change anything

 

Ive seen intake gaskets and injectors cause similar issues on the trucks.

 

I guess it could be the intake gasket, seems unlikely though as I totally rebuilt the engine about 25,000 miles ago and it's been fine so far. That seems pretty quick to be having a problem with the intake gasket. I'll spray it with carb cleaner to check later. How would I check for a bad injector if it's not throwing a code for a specific one? Normally I'd swap it with another one, but switching injectors around 4 times seems like the hard way to do it

 

A much cleaner way to check for leaks is to use a plumbers torch gas on but not lit. Super easy and mess free. I always think about damaging paint etc when people spray solvents on a engine checking for leaks.

 

Easy way to check for bad injectors or ign coils is to (1 by 1), unplug them while the engine is running. Unplugging either should make an audible difference, if no change, you have somewhere to start. If, for example, pull the plug on #2 cyl coil and it makes no difference, move it to cylinder #4. If #2 is now alive and #4 is dead, you know you have a bad coil. If #2 is still dead, you either have a bad injector or a harness issue.

I bring this up because as well as O2 sensors, dead injectors/coils can mess with you fuel trims. Fun fact, a cylinder worth of unburned air/fuel will read as lean to a O2 sensor, since you have an abundance of unburned O2 coming out of it. That would be 1 key way that O2 sensors are different in function to Air Fuel Ratio sensors.

Hope someone's suggestion here helps you pinpoint your problem!

 

Fault diagnosing these things without just swapping parts over, can be a big pita.

I'd swap over coil packs first. Then put new O2's in.. or O2's first then coils. I know, parts cost money. I have over 10k worth of engine parts that are not even on my car yet, let alone what is already on the car. When you choose this hobby it is just how it is.

 

My leak test method is a cigar and whiskey. Get the cigar going, take a big drag on it, exhale into the throttle and slam it shut.

The whiskey is because nothing goes with a cigar like a good whiskey

 

Run it in open loop and see how it acts, if it clears up then it could easily be an 02 sensor. No reason to throw parts at it until you have the problem narrowed down. If checking fuel pressure you need to do it under load a bad fuel pump won�t necessarily show at idle but under load it will for sure. If you can data log And have a wide and it will help narrow down problems easier.

 

Sorry for the lack of replies, I haven't had much time to work on the truck lately, I've been busy rebuilding the rear end on my mud truck among other things. I already bought the o2s so that's kind of a moot point now haha. I haven't been able to put them in since the regular o2 socket wouldn't fit, I'd need a crows foot socket. So I'm just gonna bring it to the shop up the road and have them replace them. I revved the engine to about 2k when I checked the fuel pressure and it held steady, I didn't try to put it in gear or drive it around though since I had to have the gauge laying on the floor so it wouldn't have been any use while I was driving anyways unless I had a passenger

 

Agree. Put it in open loop.