Hi everyone, I've not posted before, as I could usually find anything I need elsewhere on the forum. It really is a great site, lots of info, lots of knowledgeable people! Unfortunately, this time I'm coming up empty handed. The search function just can't seem to make sense of my queries. Here's the story.

The car: 1991 Chevy Camaro, 3.1L V6 LH0, Automatic Transmission 4L60E, 159,600 miles.

The problem: Engine seems to be down on power after work done. Poor acceleration compared to before.

Work performed: Cylinder heads removed and overhauled at machine shop. New timing chain, oil pan, water pump, starter, rad and hoses, distributor rotor. Intake manifold blasted clean at same shop. Lots of cleaning and new gaskets on everything that came off.

Other info: Gas in tank only a month old. Fuel injectors cleaned within last year. New distributor cap, plug wires, and plugs also within the last year. New exhaust system from cat back within last six months.

Diagnosis: Engine compression test came up with no problem, all cylinders within 5psi. Ignition timing is adjusted to spec. Vacuum test showed consistent manifold vacuum at idle. Idle seems smooth. A bit of noise- possibly from exhaust system, valve noise, or both. Transmission fluid level OK. No leakage of coolant into oil detected. Timing chain marks are indeed lined up.

Hypotheses 1: Installed timing chain off by one link. Could the crankshaft/camshaft moved during removal and installation? If so, how do I get it back?

Hypothesis 2: Machine shop did not bother to measure valve springs for loss of height. When adjusting valve lash to spec, wear/spring sag was not accounted for, so valve lash too tight.

Anyone that has any ideas, insight, or experience, your suggestions would be greatly appreciated!

 

Timing issue most likely.

Did the guys who did the heads do all that other work?

Have you had it back to them to address this issue?

 

...and here's another thing to check.

Make sure the throttle linkage is correctly adjusted. If the linkage got buggered or mal-adjusted during all that work, and the throttle won't open fully, you are going to notice a LOT less power.

Check it to be sure the throttle will open fully.

 

W.E.G., I did the rest of the work myself. I removed the heads, took them to the shop, picked 'em up and put 'em back on. Will check throttle tomorrow. Thanks for the suggestion.

Edit: Got excited, went out and checked throttle after writing the previous. Throttle operation is OK.

 

1. There was no such animal as the 4L60E in 1991. Just the 4L60 NON E (that wasn't in existence until 93).
2. What were your compression numbers? Why did you suspect the heads needed a valve job? And your manifold vacuum reading?
3. If the 3 timing marks on the cam and crank gears lined up, you're good. The engine would either run or not (dizzy 180* out of alignment due to cam being flipped as compared to dizzy position when removed).
4. Reset the TV cable at the throttle body. This MUST be done via the gas pedal once the cable sheath is reset into the body.

 

Maverick,
1. Right. 4L60. Thanks for the correction.

2. Numbers were, for cylinders 1-6, in PSI, respectively: 163, 160, 159, 164, 164, 163. Engine was cold. A little higher than the 100 to 150psi spec'd. These numbers are consistent with what it read before this work was done. I assumed it was due to carbon buildup inside the combustion chamber (which is now all gone.) Engine was running slightly rough at idle (no misfire, just a fair about of vibration especially under load). Was unable to determine cause of roughness (no vacuum leaks, new motor mounts, ignition timing ok, IAC replaced). I had the valve job done because the engine hasn't had any major work done and thought it would be a good learning opportunity. The machine shop discovered a small crack in the passenger side head casting. Vacuum was around 17 in Hg at idle if I recall correctly.
3. Three timing marks? I only see two, one on crankshaft gear, one on camshaft gear.
4. What exactly do you mean by this?

 

Your trans should be a 700R4, those were in our cars till 92.

Did you make sure to set base timing with the EST disconnected? What do you have base timing set at? Stock was 10 degrees.

 

As the transmission is most likely not the issue, I doubt it matters what it is. It is a 4L60 though.

I set base timing (with EST disconnected) at 10°, per the emissions label. I've been out there monkeying with it again, so it'll need to be set again. I'm kind of grasping at straws here since the compression test came back good.

Thanks for the suggestion, I'll keep everyone posted when I get it put back together, hopefully later today.

 

I'm assuming the check engine light is not on, have you checked to see if there are any codes that may be stored in there? Do the RPMs increase smoothly if you slowly open the throttle? Has the fuel filter been changed out somewhat recently?

How did you tighten down the rockers? I would doubt this is the issue since you came up with good compression readings, but thought I'd ask anyway.

Maybe throw a fuel gauge on it too, see what kind of pressure you're getting. If it was too low, you would probably have a misfire, but I'm just trying to get as much info as possible and rule out any possible oddball issue.

 

CEL not on. I've disconnected the battery, since I'm still working on it, which unfortunately will erase any stored codes.

RPMs seem to increase smoothly. It feels like the engine has to rev up much higher to get up to the next gear.

I'll have to see about scrounging up a fuel gauge.

I've tinkered with the distributor a bit. I'm going to rinse the cooling system flush out of the system and button her back up this afternoon. I'll do a test drive, maybe shoot some video of what I'm seeing.

I'm right at two years at current fuel filter. I did not think to change it since acceleration was fine before.

Rocker arms were tightened down to point of zero lash, as determined by rolling the pushrods until resistance was felt. Then 1-1/2 turns after that per spec. That's quite a bit more than folks here on the forum seem to use, since the valve systems on our cars are not new.

 

rockers should be 1/2 to 3/4 turn after 0 lash

do not roll the pushrods in ur fingers while tightning down the rocker nut to find 0 lash
instead wiggle the pushrod up and down till it stops wiggiling then ur at 0 lash


reason being is sometimes the pushrod will spin freely even once there is preload on the lifter

 

OK, I'm starting to suspect "valve float."

Valve float is what happens when the lifter fails to maintain good contact against the camshaft during the closing cycle of the valve's movement.

The purpose of tightening past zero-lash is supposed to be to "center" the plunger of the hydraulic lifter within the body of the lifter.

Somewhat the mechanical opposite of valve float (but perhaps manifesting in the same symptom - loss of power) is "pump up" of the lifters at high RPM. A pumped-up lifter could keep the valve open longer than ideal. I kind of doubt you have a pump-up issue with a third-gen V6 motor, but its worth keeping in mind when you are thinking about what the lifters are doing.

You followed the manual's spec when you turned the lash 1.5 turns past zero. But clearly, you are having some issues here. I like the idea of cutting that turn-past-zero in half and see what happens.

 

Even the GM service manual is wrong. It's a guarantee you will float the valves if you tighten the rockers 1 1/2 turns after free play is removed. I wouldn't do any more than 3/4 turn... I usually do about 5/8 (halfway between 1/2 and 3/4 turn). This is what I did after my engine spent 3 years on a stand in my garage and I can guarantee the lifters were empty of oil.

Yes, there are 2 marks on the cam gear. One is down (pointing at the crank mark) with #1 at TDC and the other with #4 at TDC. Not that it matters much since your engine seems to be running okay except for the loss of power (would have more serious issues if the timing set was misaligned).

The TV cable is the THROTTLE VALVE cable for the valve in the transmission, has the green end on it that mounts to the throttle linkage on the throttle body. The end of the cable sheath is in a big block of plastic that has a metal clip in it. The metal clip gets pushed towards the cable and the sheath body pushed towards the center of the car until it stops and then you get in the car (WITH THE ENGINE OFF) and floor the gas pedal to adjust the cable, which will pull itself back through the retainer body on the cable bracket.

As far as the transmission designation, the Federal Government mandated the designations for transmissions/transaxles in late 90. The 700R4 was renamed the 4L60 for 91.

 

Update! I kept messing around with the ignition timing. Even tough it was supposedly set correctly (10° BTDC base timing with EST connector disconnected), TWICE, I got in there, moved the distributor around a bit, moved the cap around a bit, and I think I'm back to where I was before! The power's back, but so is the vibration that started this whole train a'rollin. I'm going to tackle that probably in another few weeks, for now I'm back on the road! I'll start a new thread for that issue at that time.

Thank you all for your suggestions and comments.

About valve lash- I plan on going in and loosening it up a bit. Maybe setting it to 3/4 turns after zero lash. Should clear up the valve noise....?

As for the TV cable, seems to be operating normally.

Thanks again for the help, hopefully I'll see you on the next thread. I'll update this one once the lash adjustments are done.

 

Cool beans.

Vibrations can be maddening.

The PO on my car jammed a big hunk of wood between the tranny and the tranny-hump. I figure he didn't just lose a piece of firewood there. Probably was for some vibration issue. I'm afraid to take it out to see what happens.

 

Might get under the car and check the transmission mount... It could be that someone installed a polyurethane mount in the past. Which has 2 downsides...
1. It's harder than the rubber mount (transfers more vibrations)
2. From what I understand from reading many posts about it, the mount seems to have different dimensions from stock, which causes a driveline misalignment and subsequent vibrations.

There are other variables, but they can only be checked and/or corrected with the engine out of the car (engine frame mounts and flexplate).

W.E.G., the PO probably got tired of replacing transmission mounts and when the last one split, he just left it and stuck the wood in there to keep the transmission from jumping in between gears... I know I hated pushing mine back down when the mount split on it.

 

I replaced the rear mount not too long ago. Might be able to find the receipt and figure out which type I bought. Didn't seem to change much though...

Wouldn't a driveling misalignment only be noticeable when the car is moving? My vibration occurs when stationary.

 

Well, if you bought the $15 mount, it's definitely not poly, so you don't need to worry about it.

Next check would be fuel pressure and possibly injectors...