vibration, rough running at 1500-2k rpm L98
 

Hello TGO,

I am working on the last mechanical bug before my car goes to paint. I have a vibration at idle and throughout the rpm range. it gets so bad, when revving, the dash rattles(4,500 rpm). At idle it seems like just a slight vibration as if it were cammed. When you hold the idle at around 1,800 rpms it will buck and miss like its breaking up. Rev a little more, 2k+ and it clears up but vibration is still there. I have to assume the ignition system is ok. Injectors? Not sure where to start. It's just strange that it breaks up so bad when holding at a lower RPM off idle. I will start with what is in the engine and what i've already checked.
Engine:
L98, bored .030, 9.5:1, stock flywheel & balancer(both in good shape), Summit 162108 aluminum cylinder heads, stock l98 camshaft, 1.6 aluminum roller rockers, edelbrock TPI base, SLP runners, BBK throttle body, refurb'd distributor, cap&rotor, accell 8.5 wires, autolite plugs @ .045, Edelbrock headers, BBK adjustable fuel regulator @46 psi unplugged, bosch 3 22 lb injectors from ebay, tune from Scott Hansen @ tuned port induction services, powerdyne S/C @ 8 psi max, Timing at 6 advanced.

Things i checked:
plug gap - .045, balancer - lined up(new), correct flywheel, vacuum leaks - NONE, looked at plug wires at night - no shorts, with belts off didnt seem to change at all.

I would like to get a video posted. Maybe that can help. I'm sorry for the hand holding cry for help but i'm at the road flare on the front seat point...

Re: vibration, rough running at 1500-2k rpm L98
 

oh, also important info: 255 Walbro fuel pump, BBK adjustable fuel pressure regulator. 46 Psi

Re: vibration, rough running at 1500-2k rpm L98
 

Cylinder power balance test. Pull one plug wire at a time, and see if the RPM drops. I suspect a dead cylinder......

Re: vibration, rough running at 1500-2k rpm L98
 

thank you for the reply! sounds good. I'll do that at the distributor. would it be wise to do the same with injectors?

Re: vibration, rough running at 1500-2k rpm L98
 

May not be a bad idea.

Re: vibration, rough running at 1500-2k rpm L98
 

I feel like a complete idiot...i rechecked for vacuum leaks at the intake and found 6 & 8 runners leaking

Re: vibration, rough running at 1500-2k rpm L98
 

So i did a power balance test and all tested fine. ignition and fuel injectors. So i drove it to get a good example of what's happening. When cruising, in any gear, at 2k rpm's it bucks and jerks. If in 1st gear cruising its bad. If i'm in 3rd or 4th in feels like it cuts out then comes back. Not sure if the guy that changed my computer chip to compensate for deleting the 9th injector did something else or if it's a bad maf or ignition module?....ready for the road flare

Re: vibration, rough running at 1500-2k rpm L98
 

Did you check the fuel/air mixture?

Re: vibration, rough running at 1500-2k rpm L98
 

I'm having similar symptoms and I have a feeling a vacuum leak is responsible. How did you find the runner leak?

Re: vibration, rough running at 1500-2k rpm L98
 

can of brake clean with the little red straw. I just sprayed little bursts and waited for an RPM change. it would flare up when i sprayed around the bottom of #6&8 runner to the manifold.

I fixed mine but it did not cure my issue at 2,000 rpm's. Still bucks and jerks if i'm cruising through a parking lot in 1st gear.(most pronounced). even checked A/F ratio...i'm just going to remain in WOT the majority of the time.

Re: vibration, rough running at 1500-2k rpm L98
 

update: replaced the fuel injectors. during which i removed the egr valve to inspect. found the pintle is not seating all the way and there is air coming through it when i applied vacuum to the exhaust port with a mityvac. As soon as the memcal comes back for the 24 lb injectors ill test drive

Re: vibration, rough running at 1500-2k rpm L98
 

Are you saying that the ERG is stuck opened allowing exhaust to pass through?

Or are you saying the valve itself as a vacuum leak?

Re: vibration, rough running at 1500-2k rpm L98
 

The EGR was stuck open slightly. Unfortunately it did not fix my problem.


I am still having a misfire. Not that noticeable at idle but when i rev it at a stop it vibrates badly. Then when i am cruising around 2,000 rpms the miss causes a surge in cruising and shakes the car like there is no harmonic balancer all the way up to 5,500 rpms. it's a miss. gotta figure out how to isolate it. i will continue to update here until i find the issue. I've spent months on these forums and found nothing.

Re: vibration, rough running at 1500-2k rpm L98
 

i am going to focus on injector harness and ignition system. i have ruled out most of the fuel system(except harness). Now i HAVE already noid light tested the harness but that is when i disconnected then one by one. maybe a break in the line somewhere. i am going to wiggle harness with noid lights this time.

Re: vibration, rough running at 1500-2k rpm L98
 

wiggled noid lights with engine idling. all worked well. revved engine on a couple cylinders and the noid light got brighter. checked with maf connected and disconnected - no change. It's definitely dropping a cylinder. I have my valvetrain set at 3/4 turn past 0 lash. still get some valve train noise at idle. I wonder if the retro-hydraulic lifters could be screwing with the compression? vacuum gauge is steady though....lost

Re: vibration, rough running at 1500-2k rpm L98
 

After sleeping on it, i am going to recheck compression and do a leakdown. Then recheck the ignition by pulling a plug on each cylinder and also checking for correct spark plug routing(highly doubt it would be off, would have a major backfire). Then will change course and start hunting flywheel/damper for vibration. Didn't have this before rebuild with same parts but I had the flywheel machined and new clutch. Like i said before i DO have slight valvetrain noise, from what i've read on here it is normal. BUT, if EVERYTHING checks ok, then i'm going to replace the lifters with the best ones i can find.

Re: vibration, rough running at 1500-2k rpm L98
 

I think aftermarket performance hyd lifters usually make more noise than stock. I suggest not replacing them unless you think you may have a collapsed lifter.

Re: vibration, rough running at 1500-2k rpm L98
 

Thank you for the reply! Yes i checked the valve adjustment and the rockers all were fairly close to the same height. Logic has me ruling out the flywheel and balancer because it didn't vibrate(miss) before engine build. Also the breaking up(missing) between 1500-2200 rpm points to a miss. i just can't imagine it is valvetrain related. I am doing the compression and leakdown test this weekend. Then i will try a spark plug swap. Like i said, I have ruled out the fuel system completely at this point.

Re: vibration, rough running at 1500-2k rpm L98
 

I did a power balance test at the distributor cap. Had the a/c running to get as much of a load as possible to notice a change in RPM. All cylinders dropped at least 80 RPM. So can I rule out a dead cylinder? Could there still be a miss?

Re: vibration, rough running at 1500-2k rpm L98
 

Replaced spark plugs late last night just for the heck of it. Ac Delcos. Couldn't start it due to being late, will start it asap after work.

Re: vibration, rough running at 1500-2k rpm L98
 

Just preparing for the inevitable, Does anyone know a shop in Denver that can do diag work with TPI?

Re: vibration, rough running at 1500-2k rpm L98
 

UPDATE: Installed AC Delco Rapid Fire platinum plugs at .035" Vibration is at least 90% better! stupid Autolites... Anyway, it still breaks up between 1500 and 2,000 Rpms. Got an Air/Fuel ratio meter. Attached to oxygen sensor in drivers side header. Idle is stoice, Full throttle is slightly rich(good) but when i hold it at.lets say, 1,700 Rpm's it breaks up badly and the A/F ratio is EXTREMELY lean! Like barely registering on the gauge. I am thinking of bumping fuel pressure to 50 or 53 psi(at 46 now) and installing the Granatelli adjustable MAF to get part throttle richer. I see on here people hate this MAF. ANY ideas? ANY at all?

Re: vibration, rough running at 1500-2k rpm L98
 

Sounds like a case of un-metered airflow. Are you sure there are no air leaks between the MAF and blower?

Re: vibration, rough running at 1500-2k rpm L98
 

you know what, ive checked, but i am going to go through it again with the smoke pro machine. Thank you for taking the time to respond!

Re: vibration, rough running at 1500-2k rpm L98
 

NP. BTW, an o2 sensor will read a missfire as a lean condition so see if you can check your secondary waveform on a scope. Failing coils or a bad pick up / module can ruin your whole day with a blower.

Re: vibration, rough running at 1500-2k rpm L98
 

Yea!! it is picking up a very lean condition! I have an extra distributor from the boneyard. I'll swap in the pickup and module too and report back. As for ruining my day....lol...i really want to drive this damn car but can't with incorrect fuel ratios

Re: vibration, rough running at 1500-2k rpm L98
 

UPDATE! decided to hold off on replacing the distributor. found a Granatelli MAF on ebay. Put it on and 90% of the problem went away!!! i think i have to fine tune the adjustment screw but i think i nalied it! freakin MAF

Re: vibration, rough running at 1500-2k rpm L98
 

I performed another smoke test last night. I found there was leaking out of the throttle shafts. This TB is like new. I did have the smoke machine cranked up. It leaks regardless. Not sure if this can be fixed or is a problem. I sprayed carb cleaner with engine running and the idle/o2 readings did not change. This may be nothing but...

Re: vibration, rough running at 1500-2k rpm L98
 

word to the wise....do not rebuild your TPI motor. scrap it and do the LS1 swap. I now have a fuel tank overpressurization fuel boil issue. I have this car listed on craigslist. Put over $8,000 into it on top of the $6k i paid. Can't believe its still a piece of crap. Paint, interior, motor, trans, rear end, exhaust, supercharger, everything else. POS. Dodge Challenger time!

Re: vibration, rough running at 1500-2k rpm L98
 

calmed the vibration down with a pulley spacer for the s/c. the rough running only happens with s/c connected. If i take the pipe off the s/c it runs perfect. So I believe it will be in the tuning. Even though I have the FMU, booster pump.

Re: vibration, rough running at 1500-2k rpm L98
 

i have not sold the car yet. eliminated any logical internal possibilty by replacing camshaft, lifters, rockers. I re-replaced the distributor. Set timing to 10 advanced(just a test), then back to 0. It has become undriveable at this point its so bad. Also the fuel smell from boiling fuel tank(separate issue) is bs. I'm thinking kiss method says re-replace the cap and rotor and plug wires.(have less then 1,500 miles). But my gut says this is all in the tuning. has to be! everything under the hood is new!

Re: vibration, rough running at 1500-2k rpm L98
 

First, I know nearly nothing about FI. You say it runs perfect without the SC. To me, it sounds like you have weak spark. Whenever extra demand is put upon the ignition system, the spark gets blown out. How many ignition coils are on this thing? If I assume 1, then I suspect your rotor might be arcing through the plastic to the center post. I had this happen on my 95 GMC Sierra 305 TBI truck. I carry a spare rotor with me.

BTW, NEW parts don't guarantee that they are good and working.

Re: vibration, rough running at 1500-2k rpm L98
 

Very good possibility, when the OP replaced the Autolites gapped at .045" with the AC Delcos, he closed the gap to .035" and stated that it runs better.

Also, OP, are you using the correct 3/4" reach flat gasket seat plugs? Such as the Autolite 3924?

RBob.

Re: vibration, rough running at 1500-2k rpm L98
 

Very good point.

Re: vibration, rough running at 1500-2k rpm L98
 

I must apologize for not being on here for a year. I put a cover on the car and had a few people look at buying it but after all this time, i am ready to get back after this issue. I have $16,000 into this project and was defeated last year after a tuner spent 3 weeks with it and it still had the same "missing" issue while driving at 5-10% throttle(cruise). I want to thank everyone for the responses and my new game plan is an msd box with an upgraded msd coil. RBob: yes i have the correct plugs for the aftermarket alum cylinder heads from Summit. I OHM'd the wires and they are perfect(never had an Accel set bad in 20 years and 2 dozen sets i've made, but it was worth removing them and checking.

Re: vibration, rough running at 1500-2k rpm L98
 

Have you looked at the ECM as a possible cause?

Re: vibration, rough running at 1500-2k rpm L98
 

It's been almost 3 years now and not one person can help me with this. I hired a local tuner and he was under the impression it has to do with the way the supercharger and the TPI runners interact. Not sure. Its not even fun to drive. I was thinking of removing the supercharger and sell it as is. Or leave it on and sell it. May be someone smarter than me would have better luck. If anyone knows of a TPI specialist in Denver area let me know. Or if you want a nearly completed car...

Re: vibration, rough running at 1500-2k rpm L98
 

My car has a S/C and no vibration...this is not a S/C design problem the tuner mentioned.

Just a remote possibility: Take a LONG screwdriver and hold it onto each fuel injector with the car idle while holding the large end of the screwdriver to your ear and listen for the injector pulse/peck. You will know if one is not pulsing. If not pulsing wiggle the wire.

ECMs act really stupid when they are failing. I keep a backup.

Reminds me though...I looked at a 5k mile Iroc-Z about 2.5 years ago that had this weird vibration at low RPMs. Seller had no idea what the deal was with that and I did not buy the car.

Re: vibration, rough running at 1500-2k rpm L98
 

thank you for the info! I had the car for sale, had a buyer and decided to keep it. I am going to take these steps: remove supercharger completely. Sell it. If vibration and miss is still present, I am removing the L98 and parting it out for LS1 money!!

Re: vibration, rough running at 1500-2k rpm L98
 

End of this post.....Never removed S/C. Could never figure out engine rotational vibration. Possibly bad damper or flywheel. As for miss, only thing i could think of was the S/C causing weird turbulance in TPI. Anyway, doesnt matter. It went to a new home and I LS swapped my 77 T/A. Turned out great. I spent 4 years trying to figure out these issues and nothing worked. Just lost interest. My advice for anybody reading this, DO NOT dump a ton of money rebuilding a L98. Just swap to LS and be done.

Re: vibration, rough running at 1500-2k rpm L98
 

The follow up closure post to this thread. The guy that bought the car found, through a tuner, that the eprom was faulty and sending the fuel maps all over the place. Frustrating for me because i had that chip custom dyno tuned. Oh well, enjoying the new toy that took its place. So if anyone out there reading this in the future, if you have custom work done and the mechanicals are ok, have a tuner check the fuel mapping in the chip. chips CAN go bad.