Need to figure out why LX9 runs poorly

[Superdave]

Yep, it'll bolt right on.


Just remember to bleed the pressure off the rail at the schrader first.. and keep a paper towel down under the FPR when changing it. Also lube the top and bottom Orings.

[jmgtp]

Sounds good. I picked up a new FPR (55psi) last night. I'll swap it out in the next few days and try that bin out. The Impala that I worked on this past weeked was a 3400. I was jealous at how smooth it ran, and it was far more powerful feeling than my ill behaved 3500. Hopefully the new FPR is step 1 on the road to recovery.

[SappySE107]

Only if it was bad.

[jmgtp]

I gotta start somewhere.... we know it wasnt the tune and I went over the wiring more than a few times. I'm hoping the bump in fuel pressure will make some difference. I've been real lazy about a compression test but that would be next on my list if the FPR doesn't do anything. I guess I kept telling myself a 60v6 with 52k miles can't possibly have a mechanical issue but anything is possible. It sat on a stand for ~1.5 years and in the junkyard for who knows how long. I've heard of stuck rings causing problems but I've ran more than a full tank of gas through this motor, including WOT and high rpm decel, you'd think that anything stuck would be free by now.

Just to check.... since the rockers are pedestal mounted, you can't really tighten them too much right? I know on a conventional SBC if you tighten a rocker arm too much the valve would hang open and it would run awful - but on those motors the rockers are stud mounted. Since the LX9 rockers are pedestal mounted, once the pedestal bottoms out against the head thats it - can't go anymore. There isn't any lash to set. That said, I did torque them to spec when I changed the LIM gaskets. That's the only thing I touched on this motor.

[pocket-rocket]

Well..... you could over tighten, but you're going to rip the threads for them right out of the head

[jmgtp]

Well a month later and the 55 psi FPR is still sitting on the shelf. I had a few cars come in that needed repair (and I needed the cash), then I subsequently had a flare up of my sciatic nerve, a hurricane rolled through - always something to slow me down...but... tonight the FPR goes in. Since I haven't looked at this thing in ages it seems, I can't recall if I need to pull the plenum off to replace the FPR. I know it hangs off to the side by the neck of the plenum but not sure if that makes it inaccessible or not. Just wondering if I should pick up some gaskets on my way home from work or not.

Fact is that I now have a real need for this car to run well. I traded in my daily driver Mazda6 for a truck. Turns out trucks are real pigs on gas! So the more I can drive the Pontiac the better.

[WrathOfSocrus]

The stock metal plenum gaskets are reusable on the 3500, so you shouldn't need to replace them unless the rubber is torn up. It's probably possible to remove the FRP with the plenum on but I'm sure it's easier to just take it off and use a rag or something to catch the fuel that comes out.

[jmgtp]

I want to say that when I replaced the LIM gaskets I also replaced the plenum gaskets and the ones that went in were ordinary paper gaskets not steel/rubber.

I have gasket tack seal that I may apply and reuse them if they are paper. A new Felpro set appears to be $30! If I'm lucky the FPR will come off without removing the plenum at all.

[1988GTU]

FPR should be replaceable with UIM still on.

[jmgtp]

Only took about 10 minutes to swap. Everyone that said it is possible to do without removing the intake - you are 100% correct.

I verified 55psi on the rail with the vacuum line unplugged. I burned the bin that was setup for 55psi on 3400 injectors and I instantly had a smoother idle - not perfect yet, but idle was a little shakey before and now it is noticeably less so.

Took it for a drive. Drivability is good running through the gears. On the highway in 5th it still gets a mild case of the shakes under 2500 rpm (roughly under 65mph) under any kind of load.

WOT is fairly unimpressive. Perhaps it could take down a 3.1 GenII motor. It does have enough grunt to squeal the tires (245s at that) from a 5mph roll if I stomp on it.

I think tonight I will finally do a compression test to try to determine if this is in fact mechanical.

Other thought - could this have anything to do with the harmonic balancer? I'm using a 3400 balancer of unknown mileage - visually it looks OK.

[jmgtp]

Fixed it! Finally it runs well!

The compression test revealed 175 psi on all the cylinders except #4 which came in under 50 psi. I pulled the rocker cover and discovered that there was NO pushrod on the #4 intake valve! I found it laying down in the lifter valley. Retrieved with a magnetic pickup tool, installed, ran a compression test and had 175 psi on #4.

I replaced the LIM gaskets when the motor was on the stand. I must not have had that pushrod seated correctly when I put it back together. Man do I wish I ran a compression check earlier! I was too hung up/stubborn in thinking a 50k motor couldn't have anything wrong with it internally.

It is running very well. Idle is smooth (though it stall occasionally if I clutch in or load the engine at idle by turning the steering wheel.) It is SO much more torquey now that if I lay into in 1st the 245s up front are hopeless. Driveability is great. I'm sure a tune for this particular setup will sort out the stalling, improve the driveability even more and get a bit more top end power. I almost forgot to mention that it sounds entirely different. It sounded like a sputtering mess before, which I thought was a hole in the exhaust. Now, it sounds pretty mean through 2.5" pipes, no resonator and stock mufflers - best part is that it is still very quiet. I can't wait to sort the last details out and start driving it!

[mfuller]

Very glad you discovered the issue.
Wonder how the pushrod "fell out"; are there guides on the 3500 LIM gaskets like the 3400 LIM gaskets have?

[jmgtp]

I noticed that particular pushrod didn't seem to have any 'guide' in the gasket. I thought it was weird but I looked at the other pushrods and some did and some didn't. It's hard to screw it up anyway since you can feel when the pushrod is in the lifter cup and you can see/feel when it is in the cup on the rocker arm during install. Not sure what went wrong but sure seems like it was human error on my part somewhere along the way. Only wish I had checked it sooner.

Needless to say I'm so happy it runs well since I need something that can get half decent gas mileage. Only a few loose ends left before I can drive it: getting the tune a bit closer, hopefully get the a/c to work (though I think it lost its charge), and giving the car a bath! It has 2 years of garage dust on it. It needs to be e-tested again. I had it done a few months ago and it failed on the nox. I dont know if the dead cylinder contributed to that but it certainly didnt help. I re-tested with a new cat and while it improved it still failed on nox. So, ultimately I think I'll need to install an EGR and that should get it to pass.

[SappySE107]

Glad you finally found the physical issue that kept me from properly tuning your car

[jmgtp]

I know...I sent you an email about it last night.