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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. (The LH0 is a pushrod engine with hydraulic lifters.)
Anyone that has any ideas, insight, or experience, your suggestions would be greatly appreciated!
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It’s definitely possible that the shop didn’t measure the valve springs and now they are not opening as much as they should that would cause noticeable power loss for sure
I don’t see how valve lash could be the prob. With a pushrod/lifter type of setup the valve adjustments are very non-critical. It’s been a long while, but I recall the adjustment method was something like tighten down the nut until the follower isn’t loose, then 2.5 more turns.
If the compression is okay, you can probably rule out problems with the valves seating or valve timing or the cam or followers.
What’s left? Ignition timing, or could it just be your imagination?
After the compression test I’m grasping at straws. I’ve checked the ignition timing twice but I’ll be doing it again today.
I know it’s not my imagination, because I’ve never been embarrassed to drive the car before. I used to pass at least some cars on the road. 😉
Hmm, well a few lesser possibilities:
You stuffed a rag into the intake or exhaust manifold, and forgot to remove it.
An intake leak, although that often gives you a error code.
The gas pedal is misadjusted, so you never get the throttle fully open.
Fuel injector cables switched around.
As an objective test, you might try looking up the 0-60 time for this car in like an old Road & Track review, and compare it to what you’re now getting.
First of all I doubt your problem has anything to do with valve spring pressure or rags stuffed :angry: in your engine and the adjustment procedure is zero lash plus a three quarter turn.
Are you sure you have the crankshaft and camshaft marks lined up correctly as one tooth out can cause the issue you explained.
I would connect a vacuum gauge. You should have at least 20in. vacuum at sea level. If not, advance the ignition timing to see if the vacuum increases. If the vacuum increases, back off the distributor until the vacuum just starts to drop and tighten the distributor.
Drive the car to see if the power level returns. If the power returns I suspect the outer ring on the balancer has slipped or there is little or no timing advance on acceleration.If the vacuum at idle is low or did not increase or by little when advancing the timing I suspect the crank and cam are not timed correctly.
Anytime you have a lot of work like that done and you have a problem afterward, you need to go back and recheck the work. In this case, the machine shop did the work. I’m assuming you did the assembly work. Regardless, it’s a good idea to go back over everything to make sure it’s all hooked up correctly. As for what to look for, timing was the first thing that came to mind but you said you covered that. As to your question about the cam gear being misplaced, not likely. There is a locating pin that aligns the cam with the gear. As long as the timing marks are lined up, mechanical timing should be good. It could be something they did with the heads but I don’t think it would have anything to do with valve springs. Did you install a new cam and lifters? If so, perhaps it’s not the right cam for the engine. With that amount of work there are a lot of possibilities for problems. I’d do a good once over to see if I could find anything out of place. I also like the idea of checking the engine vacuum. That can tell you quite a bit about the condition of the engine.
Keep us posted.
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