Cylinder Wave Form Questions

[aball4620]

So this project has been a fun - if not frustrating - learning experience. I replaced the heads with new several months ago after blowing and injector o-ring. I thought I maybe had a small head gasket leak, and the '88 2.9 was still prone to cracking. Put it all back together and doesn't run well. I finally took it into a mechanic to have some diagnostics run and he is as confused as I am. I'm attaching the cylinder wave form readings here for your thoughts. I'm also developing a working theory as to why things are not functioning correctly and I wanted some feedback. Current symtoms are hunting while idling and not wanting to rev above 2k. Will barely move and certainly isn't driveable. No codes. Every - and I do mean every - component of the fuel system has been checked and tested. All sensors are new and confirmed working. The ECM is confirmed working and has been tested.

New heads, gaskets, lifters, pushrods, etc. Top end rebuild. Also a new distributor, coil, wires, plugs, etc. Everything has been done correctly and confirmed both through this forum, Chilton, Haynes, and then finally by my mechanic. This thing has always run rich and always been a little gutless. There has almost always been a little wispy white smoke out of the back even though I wasn't losing coolant. Or at least not quickly. The guy I bought this from was an absolute hack and I first noticed that when I blew a fuel line between the reservoir and the rail pump because he had forced the wrong size clip on backwards. So with the rebuild the timing was set at 10 degrees advance, the harmonic balancer confirmed attached correctly, and the new distributor and ignition module checked, and rechecked, and rechecked. The mechanic thinks that cam timing was maybe off before tearing into it and I didn't catch it. Didn't even occur to me to check the timing before the tear down. And that with reassembly we're seeing that 27 psi on the exhaust stroke, meaning that both valves are closed and creating the back pressure that is keeping this thing from revving correctly. For what it is worth, he was pretty sure it was a plugged cat/exhaust before pulling it off to realize it had been knocked out years ago. Unplugging the o2 sensor changes nothing. The mechanic is stumped and he thinks the only logical next step is to install a new timing chain correctly (might as well do the water pump while I'm in there). What do you think? He's excited for me to tear this thing apart again and take pictures.

My working theory is that the PO replaced the timing chain but didn't do it correctly, and then fiddled with the timing advance to get it to run. Me, having installed a new distributor, covered up the tracks in the snow by doing it correctly as the book says. All of the other issues I've had over the last couple years could be because of the cam not being timed to the crank correctly.

 

[adsm08]

Well it is hard to find a cam/crank timing error without having both signals on top of each other, and there is no crank sensor on a 2.9. A compression test could point to a possible timing issue.

The only sure way is to pull the front cover and look at the timing marks.

 

[aball4620]

Well it is hard to find a cam/crank timing error without having both signals on top of each other, and there is no crank sensor on a 2.9. A compression test could point to a possible timing issue.

The only sure way is to pull the front cover and look at the timing marks.

Is it that simple that there are marks beneath the cover? I've never opened the timing cover on this engine.

Anything else you could glean from those results?

 

[adsm08]

There are marks beneath the cover. It would have been somewhat simpler while the heads were still off, but it is still not too complexicated.

Before starting you need to identify the compression stroke, then bring your engine around to TDC on cylinder 1 compression.

Remove the timing cover, which may be a bit of a chore, and then you might have to clean a bit to see the markd you need.

On the cam gear there is a dimple. Just a simple dot stamped into the pulley. There is a similar one in the crank gear, usually close to the woodruff key. When cylinder 1 is at TDC compression the dots should point at each other.

Since the 2.9 is a non-interference engine you can just remove the chain and make adjustments as needed, but if they are close, but not right (you can be off about half a tooth either way and not cause issues) you may have a problem with too much slack in the chain too.


I did not see anything of much use on the o-scope recordings.

 

[aball4620]

Awesome, thank you. I've also read about the tensioner breaking apart and ending up in the oil pan. When I was initially considering the timing I was wondering if I had somehow reversed the crank when setting the rocker arm assembly and skipped a tooth. The mechanic was thinking that with it at last idling that was an unlikely scenario, but it sounds like perhaps the tensioner could be the culprit too.

This ran okay prior to the tear down, so I'd be pretty surprised if it was just slack in the timing chain. Any thoughts on the timing chain/sprockets being set up incorrectly and me setting the timing with the new distributor to factory specs causing issues? It sounds like with a woodruff key there is not really an incorrect way to install the sprockets so maybe shoots down that theory.

 

[adsm08]

Not sure about the timing thing. I guess it's possible.

You can't install the gears wrong, they are keyed. You can have the two shafts not timed to each other correctly though. That is very possible.

 

[aball4620]

Great, I'll open it up and see what we have. I appreciate the clarification.

 

[aball4620]

So this project was shelved for a bit but I've been back at it. I ended up replacing the timing set and the water pump (because, why not?) instead of relying on diagnostics to show even a small issue in timing. Turns out the crank-cam was set correctly, and while there was a little slop in the timing chain the tensioner was working correctly.

So I've gone through and played with the pre-load on the valve lash. The manual says 1.5 turns (or 2mm) past zero lash on a cold lifter. So interestingly, it runs like absolute garbage at 1.5-1.75 (allowable "normal"), spins out a little over 1.75 (keeping the valves open so no pressure) and runs "best" at about 1/2 turn past zero last. I've also taken it down past zero to the point of rattling - not so good - and at 1/4 turn increments all along the 0-1.75. Half seems to be the best but it is still producing the same symptoms. I'm not sure what to do with that. Thoughts? We're getting close to pulling out the old heads and lifters and sacrificing a new gasket and bolt set just to see if there is something going on with these heads. I cannot imagine these new heads would be so far off that I'd be getting such un-drivable symptoms, but.... And my understanding is that exhaust valves can only fail open as the rocker arm forces them open, and yet getting a high exhaust pressure reading.... Is it possible that a hydraulic lifter could have failed and collapsed such that the rocker arm moves a bit but not enough to overcome the valve spring force?

 

[Uncle Gump]

Have you put a vacuum gauge on it?

Plugged exhaust is a possibility.

 

[aball4620]

Yeah the mechanic that did the wave form testing said that was the first thing he checked after the timing. Everything checked out. The PO knocked out the cat and there are a couple decent size holes between the cat and the muffler. Other than the cat and muffler would there be any other place for an exhaust to plug up?

 

[alwaysFlOoReD]

Was it ever in a field? Maybe a mouse nest?

 

[aball4620]

Interesting idea.... mouse crawled up inside the exhaust manifold through one of the holes and plugged something up with a nest.... The bolts are new since replacing the heads, so should be a simple enough inspection. It has sat on and off over the last year while slowly trying to problem solve the issue, but since the heads were installed it hasn't really sat for very long.

Not being super familiar with how a mechanic would test for exhaust pressure, I'm assuming it is just a gauge in the o2 sensor hole. Given that he got normal readings when measuring exhaust pressure, but the cylinder wave form shows a 27 psi exhaust pressure, would that mean.... shouldn't that indicate the issue being in the head somewhere and not downstream?

 

[adsm08]

Correct. An exhaust back pressure test is done with a gauge that screws into an O2 sensor bung. But that hole is not right against the valves, there is space for an obstruction.

I am not familiar with this "cylinder wave form" test, and I'm not sure how it would show exhaust pressure, or determine how much of it was there, but yes, you are starting to point to an issue in the head.

Simpler than swapping heads just to test would be pulling just the valve covers and turning the engine by hand. Since it's a pushrod engine you will be able to easily observe the valves opening and closing.

 

[aball4620]

Everything seems to be moving just fine when doing the lash adjustments. Unless I have a broken spring inside of a lifter than isn't quite able to overcome the valve spring tension.... which seems... unlikely I guess. But this thing isn't breathing correctly and it has to be some sort of mechanical issue here. I'm just not sure what it would be with the exhaust pressure being what it is and the exhaust valves *appearing* to be opening correctly. Seems like a blockage in the exhaust header caused by a mouse nest would burn up after a few minutes of idling. No?

He used a picoscope oscilloscope. I'm not deep enough in to pro league mechanics to know much about it, but that is how he referenced the charts at the beginning of this thread. Quite possibly not the correct term.

 

[alwaysFlOoReD]

Mouse nest is a big long shot. They do like constricted places but usually pick warmer places. Had one fill my fireplace ash vacuum cleaner to the brim with grain, vacuum was in the outside shed.