Recently rebuilt L98 Engine runs great in general except for one specific problem-

When the car is idling and you are sitting in the drivers seat, you can feel a random pulse that will usually occur every 1-3 seconds. This pulse can be felt both in the seat and in the steering wheel.

Based off of that information, one would probably guess that it is a fuel injector, however it gets better-

When you are in park and you press the accelerator down to about 1500 rpm, vibration still occurs! It comes and goes in a wave like manner where it sort of shakes the car and becomes louder, then in 2 or 3 seconds flattens out and goes back to normal. Then back to vibration. Occurs when engine is hot or cold.

Something else that we noticed is that when we had a timing light on the car, it would stay consistent and then all of a sudden jump way off, and then come back to normal again.

I have taken it to a few different places and have formulated my own opinion, however I would like to open it up to you guys as you have much more experience with this specific engine.

Any suggestions are appreciated. If you need more info to diagnose, let me know.



P.S. Engine seems to run the best when it is timed at 0 degrees versus 6 degrees before top dead center as it calls for. Does anyone see a problem with that?

 

Sounds like a Vacume leak to Me.
Take Care, -John

 

Any News?
I have seen this as a vacume leak mostly on brake booster hoses. They are cracked not really tore-up badly, usually toward the booster end. The motor moves a little and when the driver's side of the engine eases-up a tad...That opens the leak and the engine surges for a second. When it comes to rest (driver's side of motor down) it does ok for a couple of seconds.
This happens hot or cold and is more pronounced at lower rpms. This does not have to be the vacume booster hose...That has just been the most usual place I have found it.
If this sounds like your problem, look for any cracked/dry-rotted vacume hoses.
You can take a can of Ether-based 'starting fluid' and spray short burts around the various vacume hoses or anywhere vacume can leak (like intake top, bottm, runners, plenum, throttle body...Etc.). Move the hoses around a little while giving the bursts of starting fluid. With the engine idling, you will hear the engine rev-up and surge when you find a leak. Since your engine is already surging, you need to visually exam the hoses first. You can also, notice between the surges you already have a surge. If you do, Spray a light steady shot in the same area. If the motor revs and stays revved while spraying, You Found It.

I Hope this Helps, You All Take Care, -John

Fuel injected motors also have a small DC motor on the intake assembly, these can do the same thing. I have usually found these to be the culprit on Ford motors more than on GM. If other options fail. Look in your shop manual and find your 'idle-control'/controlled vacume leak and tap on it with a large screwdriver. If idle smoothes out, There is your problem.

 

John,

Thanks for taking the time to answer my question. I will try that out over this coming weekend (December 1st and 2nd) and let you know.

Also, if you have the time, any idea about the timing jumping way off ? Where is your timing set to before the ECU messes around with it?

I also will add that it is sometimes quite rough to start, and other times fires right up.

Thanks again.

 

Could be a vac. leak, but since it's been recently put together I'm thinking valve lash....have you checked to make sure there set properly? My 87 runs best at 8*.

 

Check everything and let us know. My under-hood decal is gone. I understand yours and mine are suposed to be set at 6* BTDC (factory). LIke others have said, Individual Cars like slightly different timing. The Factory does not know what grade of gas, what your elevation is or any of the other (temp, humidity...etc.) variables are. That is why they specify a 'slightly conservative' static-timing figure.
You need to unplug that 'extra' plug on the HEI. That's the computer control and is what the computer uses to vary the timing (and will drive your timing light crazy)!
Go over the car and be vigilant! You will find the trouble.
If you wish, you can email me or ask here, and I will open the books to let you know how to set the valves (they are set at TDC and 180* out, to set all valves). If you find this to be the issue. That is a 'static setting' that is in GM Tech manuals. I will also be happy to try and look-up anything I can in the FSM's for you.
Don't get down on this issue, it is a simple road-block that you simply have to pass-thru.
You All Take Care and Be Safe, -John

 

Have you had the time to check her out?


Let Us Know, Take Care and Be Safe, -John

 

Unfortuneately, my schedule has kept me from being able to look for the vaccuum leak, however, I would like to update you on a newer problem.

Although I use Premium gas in the car, a pinging noise can be heard once in a great while after the car has been warmed up for a while at about 3000 rpm. The ping lasts for 2-3 seconds and then stops after I take my foot off of the accellerator. The most recent time that this happened, it threw a check engine light, from which I will update you guys with a code for additional info. Any initial ideas are appreaciated!

Thanks