2.8 runs rough after rebuild

[Sewerrat]

Hello, I've got a problem with a motor I rebuilt that is running rough and vibrates over 4000 rpm. It's a 2.8 V6 mpfi pulled from an 85 Firebird. I checked bore and crank specs and they were well within std. specs so new rings,brgs,oil pump, and gaskets replaced. The heads were redone by a reputable machine shop in my area. A stage II performance cam as well as Timing components were also installed. I've properly broken in the cam as specified by the manufacturer. This engine was installed into my 88 camaro to replace the original, tired, oil consuming 2.8. Both engines are code "S".

The engine idles with 15" of vac. and runs alittle rough. It accelerates well and no engine codes. I've played with base timing which did not help. The vibration I'm experiencing on the higher rpms doesn't really feel like a miss but more like when your driveshaft is off balance. It is not the drive train because you can feel it while in neutral and vehicle is parked. Listed is the things I've done which did not improve the condition:

Replaced clutch assy. thinking it may be off balanced.
Double check my timing marks on cam/crank gear.
Reset my valve lifter preload to see if to tight or loose.
Played with base timing.
Played with TPS idle voltage from .4v to .6V.
New cap, rotor, wires, plugs as well as checked for crossfire.
Checked for vacuum leaks.
Scanned O2 sensor for proper fluctuation.
Checks compression. all are even at 165 psi.
Replaced harmonic damper with new.

The performance cam I got is from Anderson Racing Industries in MO. and they state it's a computer compatible cam but I'd like to list the specs for you guys and tell me if you think maybe I need a retune...Dur@.05 int 204, exh 214/adv dur int270, exh280/lift int.420, exh.443/ lobe center int107, exh117. I'm totally stumped and ran out of ideas and patience as well as $$$. Any advice is greatly appreciated. sorry for the long post..

Mike

[ForcedFirebird]

Isn't the 1985.5 when they swiched from a balanced flywheel (externally balanced crank) to internal, along with the main journals being larger? If you used the 89 flywheel on the 85 motor is the only thing I can think of that would give you a weird vibration.

[Sewerrat]

Yes I thought about that because a website mentioned of the 85 being a different balance while the 88 was a neutral balance. I checked to see if both engines were the larger journals and both came up with the same bearing part numbers then the earlier 84 and down. Prior to my rebuild I posted this in my Camaro forum and members mentioned the difference shouldn't be a problem but I'm thinking differently now. The reason I used the 85 firebird engine is so I could swap the two engines in a weekend and reduce the down time. Now I think I'll be rebuilding the original engine and putting it back in if the bore and journals are good.

Thanks for your input.

[ForcedFirebird]

It would take only a few hours to change the flywheel to be sure. I just would follow the rule "use the flywheel that came with the crank you are using".

BTW: Welcome to 60*v6 .

Are you a member of Thirdgen.org? I'm firstfirebird there.

[geoffinbc]

Since you measure the bearings on the new crank you will be able to tell us if the crank had a circular mass in the middle. All internal balance cranks had a reluctor ring cast into the middle of the crank.

This is an internal balance crank an external balance crank will not have a ring.

[1988GTU]

[ForcedFirebird]

The RWD crank will have that same wheel cast into it, but has no notches. I have a crank from a FWD, and RWD both from a 1990 (internally balanced).

Are you saying the early crank has no wheel? What is in it's place? Pics?

[Ortie666]

did u put all of the caps back on the rods they came off of then all of them back into the same holes?

[Sewerrat]

I did mark all caps prior to disassembling and reinstalled the same. BTY, I can't remeber if the 85 block had that reluctor ring on the crank but I'm going to pop off my 88s oil pan to see if that engines has it. Thanks for all the input guys at least I'm getting somewhere. I should have tried running the 85 flexplate when I did the clutch (thinking the clutch may be off-balanced) just to see if the engine would run smoother. I'm probably going to end up rebuilding my 88 engine and putting that back in. I didn't realize the differences the 2.8 has. My first set of piston rings were wrong. Both compression rings thickness were different compared to mine as well as the timing cover and oil pan.

[sharkey]

85.5 was the change over to the large journal crank, however it was NOT the changover to internal blanaced cranks, that was 87. my 86 cavy uses an external balanced crank. 87 was when the fwd changed over to dis, therfore needing the reluctor wheel. instead of having 2 different cranks, gm changed to internal balanced on all motor, weather it had dis or not.

easy way to tell if its a large journal or small journal is the rear main seal. if its a 1 peice rear seal, its a large journal crank, 85.5 and newer, if its a 2 peice seal, its the pre 85.5 small journal crank. either way, if your flywheel doesnt have a counterweight on it, you best bet would be to pull the trans back out and change the flywheel for one withe the weight.

[Sewerrat]

My car is a 5spd but the 85 engine was from an auto. I noticed a large counter-weight on the flexplate so that must mean it's external balance, right? I tried searching my local auto suppliers but can't seem to locate an 85 flywheel. If I did find an 85 2.8 flywheel, will I still have to have it balanced?

[sharkey]

no you wont. if the flywheel is for an 85, it will be just fine

[Sewerrat]

I pulled the oil pan off of the 88 2.8 and sure enough the cranks has that large ring like you described. I'm 99% sure I did not see that when rebuilding the 85 engine. I think I would have remebered because it's shiny compared to the rest of the crank bobs. I can see now how much off balance it can create using the wrong components. I quess I'm old school sbc (pre87) so I've gotten to used to assuming parts are interchangable. A lesson well learned.

So next request is can someone help me find a flywheel supplier for the 85 engine. I tries searching and found nothing. Thanks

[Sewerrat]

Just to update. A local Standard Motors supplier found a reman flywheel for me at a reasonable price. It has the counter weight on it. I bolted it in and BAM.. it solve the problem. I was so happy I almost came! Thanks so much for all the help guys.

[ForcedFirebird]

Cool Beans!