Back in March, I pulled my 89 L98 apart due to 125,000 miles and obvious valve seal problems floating by when I stopped at a light. I have run the car very hard for many years, so I knew it was better to go all the way instead of stopping at a valve job.

I installed a balanced LT4 bottom end, picked up a set of Fast Burn heads and a mini ram intake and slapped it back together. The heads were milled down to give me about 10.8:1 compression. During the rebuild I also installed a BeCool radiator and an aluminum single mass flywheel.

I put the engine in and began driving the car and putting miles on it. While driving I noticed that the quality of the tune was not that good in the mid rpm levels (2900 to 4200 rpms), but at the low end and the high end the car pulled fine. I attributed this to the significant increase in compression/better head flow and that the timing tables probably need to be tweaked, but due to the fact that high RPMs and low RPMs seemd fine I wasn't too worried. (I also monitored the engine at this time and saw no knock/spark retard, so I was thinking that it doesn't have enough timing.)

After approx. 1,000 miles I switched back over to Mobil 1 from the dino and took the car to a track event. The car ran great! I ran in the top run group and there was not a car there that could pull me in the straights, C6 Z06s, stout LS2/3s, a Corinthian 67 427. Needless to say, I was eccstatic. Engine temps during the sessions were stable, peaking at 220 water and 270 oil, but that's full throttle running for 20 minutes straight on a street car and aside from the new radiator, there are no additional cooling helpers.

I take the car home, switch the tires and brakes back to street and start occasionally driving the car on the street. While driving one day, the speedometer quits working. The oil pressure gauge is also reading 80 lbs prior to cranking (works fine when the engine is running, but defaults to 80 instead of 0 with the key on.) The tachomoter is still working, but it has stopped digitizing the signal to send on to the ALDL so the ECM is reading 6350 RPMs on the serial data port. (That shouldn't matter because the ECM uses the exact RPM directly from the distributor for operation.) Basically the gauge cluster is in need of professional help.

Back to while driving the car, the lack of mid range power is bothersome but time is precious these days and I don't have time to mess with it. As time goes on, the car begins to die when coming to a light trying to idle down too low, and then other times idles at 1500 when I pull up to the light. I replace the IAC... seems to help, but its not running like it should.

It is also developing a miss at high RPMs. (5500 and up.) Must be the tune... The car has new cap/rotor, wires and plugs.

Finally get time to log some Datamaster runs. It's a nice hot day in Houston, early June, probably 95 out. Datamaster is showing a lot of knock/spark retard under a load. Performance is at its worst so far (obviously). I make some chip changes and don't get any better results, I must be crap at this.

Now I send off to PCM for Less and get a bin file from them after describing my engine parameters. It arrives and I find it to be almost identical to my orignal chip (and here I thought I didn't know what I was doing...) I go to log some datamaster runs of the PCM for less chip and send off and the car is running very poor. It's trying to idle at 400 RPMs, which isn't likely with my cam, and gives knock retard at the first sign of throttle. (keep in mind I never hear pinging/knocking) I can get it to show knock retard sitting in the driveway and flooring the engine (not reving it up real high, just the fact that throttle goes 100% it starts registering knocks). At 60 MPH, I can drag the brakes and floor it in 6th and it almost comes to a stop in the middle of the road. The ESC is pulling as much timing out as it possibly can.

The Dilemma: I can't send off bad data and expect to have PCM for Less provide me with a good chip.

1. I have tested the knock sensor and the ESC and found them to be functioning, so I can only assume they are functioning properly.
2. The engine is not running hot, but it seems to be running warmer than it was. Can't tell if that's a symptom of the poor performance, cause of, or a side effect of the increase in environment temperature the past few months.
3. Compression test on the engine shows to have 170# give or take a few on all cylinders (seems high, but it's got 1.5 points more compression than stock).

Do I have too much compression? I hear of people running 11:1 on pump gas (premium of course) all of the time. The factory shipped LT5s out at 11:1, LT4s and 10.8:1 and LT1s and 10.5:1. I don't think that reverse flow cooling does that good of a job that my engine wouldn't be able to run the same compression. And like I said, it's not running hot. I know that deposits in the combustion chamber can cause hot spots that lead to pre ignition when the engine gets warm but this thing will get spark retard when it's first started up, not to mention that I don't usually buy crap gas, so I shouldn't have an issue with deposits with this fresh of an engine anyway.

Is the dash contributing to the problem? I can't imagine why, but I've already pulled out all of my hair and am beginning to draw straws... I certainly hope that it's not because I am in the process of building an analog dash panel and have already bought most of the components...

There are no codes in the computer. I created one by disconnecting the ESC just to make sure the computer was working and it set code 42.

Car Specs:
350 cu. in, LT4 crank/rod/pistons, internally balanced w/Fluidampr
GM Fastburn Heads, LT4 valves/springs/retainers
Comp Cams 276HR-14 (220/230 duration, .510/.510, 110 CL, 114 Angle Separation)
MSD Pro Billet Distributor PN8366
30 lb/hr Ford Racing Injectors M-9593-BB302 (Just installed, but hasnt' changed the way the car runs)
TPIS Fastburn Miniram
Hooker long tubes, SLP Cats, LT1 Exhaust System
Fidanza Aluminum Flywheel, McLeod Clutch
3.90 Gears


Anybody experienced this type of heartache?

Suggestions Wanted. What am I missing???

 

exhaust restriction?

 

I would start with the simple things first. The extreme detonation leads me to start with :

Fuel Pressure
Fuel Delivery (injectors, regulator)

Then go to:

Spark plugs / wires (any arc-ing?)
Ignition System
plugged cat(s)
Vacuum leaks
compression check

Good luck!

 

The lack of an EGR Valve is contributing to the high combustion temps resulting in the computer retarding the timing.

Also, what's the fuel pressure and what fuel injectors are you using and have you checked them?

 

EGR is not used at WOT so it will not contribute to knock retard at WOT. I would check the fuel delivery (as mentioned above).

You should not have any issues with 10.8 to 1 compression.

 

Work has been hectic and I haven't had time to get on here.

The MSD Distributor seems to only want to be in the range "rotationally" where the plugs point straight backwards. My experience with older HEIs has been that you can drop them in wherever you want as long as you have the rotor pointing to the right terminal and you set the timing. So I restabbed the distributor with the plugs pointing straight at the firewall and the car runs better, but the poor performance is still there with the knock retard.

I've checked out the fuel system and it's tip top. Brand new 30 lb injectors, brand new filter, new lines from the filter to the rails, pump is fairly new and is keeping fuel pressure where it is supposed to be.

I will drop the exhaust and test it with the open headers this weekend. If that does it, it will get new cats. If it doesn't, I guess I'll have to tear the thing apart and put in a thicker head gasket...

 

ain't 30lb injectors a bit to much? Are these FMS injectors?

 

okay so you are running FMS injectors. you know they are rated at 30lbs per hr. at like 36psi don't you? so if you still have stock GM pressure they are probably at like 35lbs per hr. I think your better off with ford 24lb per hr. blue tops. they will deliver about 27.5lbs per hr. at GM's 43.5psi

 

Actually its 39.5 psi according to Ford, which is where I have the fuel system set.

LT4 takes 28lb injectors from the factory, so running 30s and having the chip set to run 30s shouldn't be an issue for my car.

I had 24s in it before running 50 psi just to keep it from being lean. If the exhaust doesn't work, I'll start looking at the injectors again.

 

ok then I say its either your cats plugging up or distributor related.

 

Hope you get that vette back up to snuff. Here's some info that may or may not be of help:

As to those thirty pound ford injectors, I believe ford rates their injector flow at a different pressure, so you maybe comparing apples and oranges. However 30 lbs, or any injector approaching 30 lbs, may be a bit big for your application.

I recently purchased a set of Bosch III injectors for my 96 LT4 from FIC. the conversation I had with Jon Bonner was most interesting. LT4 injectors DO NOT flow at 28 lbs per hour; they flow at 26 lbs per hour.
the original pre production engines did have 28 lb injectors; however the factory felt it had to detune the LT4 because the preproduction engines were producing more power than the LS engines in the soon to be introduced C5. the detuning was accomplished by ADDING material to the inside of the intake runners and reducing the injector flow to 26 lbs. Unfortunately, by that time all the literature containing engine spec's had already been published and the misinformation that LT4 engines used 28 lb injectors was established C4 mythology.

Obviously at one point your engine was running really well and something must have gone bad. I think the obvious culprit could be the catalytic converter. My reading of C4 history points out that in the glory days when C4's ruled showroom stock racing that the catalytic converters were removed to avoid overheating (presumably of the cat itself). I think your car's stellar preformance at the track overheated the cats and they melted (or partially melted over) the matrix, restricting the exhaust flow. And things are probably going to continue down hill from there until the engine quits running (or you can't stand to drive it) or the matrix breaks up completely and is blown out the exhaust (at which point it will run better, much better).

This is just an observation for which I have no imperical data to support my opinion, but a high compression engine may be better served by a cam with more valve overlap; say on the order of 112 degree lobe seperation angle. But since your engine was running so strong this probably isn't an issue.

Best luck. I'm willing to bet the cure to your engine's ills is going to be something simple.

 

Id get the car tined in real time, mail order can only get so good.

 

Are you SURE the knock sensor is okay? Amen. more likely, too, if the cats have to handle unburned fuel from a modded engine and non-optimal tune.

Is the ECM fuel trim bottomed out?

Search for vacuum gauge tests for bad cats.

 

when cats are subjected to raw fuel they melt like another guy said, I had one melt down on a 84 trans am when the car was new. It started out feeling like a miss in the engine then progressively got worse through the day until the car could only go about 20mph tops.

My friends 1990 vette went through the same thing however his car acted differently. His started surging and couldn't pull past about 3500rpm. Exhaust never glowed on his car either BTW. But he always heard the motor struggling to suck in air he described it as sounding like a vacumn cleaner. Car even passed calif. emmisions like this.

during his car progression the car got quieter as well.

 

Well... I took the exhaust loose from the headers and saw that one of the cats was already empty and the other was just fine.... I dropped the exhaust and cut off both mufflers and hogged out the one that was fine. Reinstalled the exhaust system and started it back up.

NO CHANGE... its not the cats.

I read up on the MSD forum about my distributor and read where a lot of people are having similar issues and MSD recommended a ignition module change. So I changed the ignition module, (which I found to have a crack in) and cleaned up the mag pickup. While I was at the parts house, I went ahead and picked up a new O2 sensor and put it in, even though there was no evidence that the other was not working properly. Fires up and runs now with the flick of the switch... I never even hear it crank.

With Datamaster hooked up, it's very obvious that the IAC I just purchased from NAPA isn't working. It sits at Zero even though the car is idling below where the chip is calling for. I've had to screw the throttle stop on the throttle body out just to keep it idling. Nothing like a wasted $125...

The car seems to run much better (aside from the idle).

I revved the engine up to 3500 on the lift with the hood up and let it sit for a while. The headers slowly turn orange, then bright red. I had the fuel pressure gauge on making sure I have plenty of fuel, and so I pumped the pressure up to 50 psi, which essentially makes the injectors 34#ers, and the headers are still glowing, so it can't be a lean condition... Is it possible that the headers are glowing because there is too much timing in the program? The initial is set correctly.

In the GM Performance parts catalog and in several magazine tests of the Fast Burn Heads, they all state that the total advance should be 32 to 35 max. I haven't gone into my tables yet, but can a few additional degrees make that big of a difference? I can imagine with the car at 3500 RPMs in idle, there is little load on the engine so the computer is probably giving it 41 degrees advance (if I remember the table correctly)... they're not glowing just a little either... they're red hot after a minute or so. Coolant temp climbs quickly like this as well so the combustion chamber is really hot as well. After slowing the car to idle, it seems to cool back off at a reasonable rate. I've always attributed glowing headers to increased exhaust temps due to lean mixtures, but can too much advance cause this as well?

 

glad to see improvement.

The reason 32-35 is the max is that the fast burn head intentionally creates turbluence in the combustion chamber. Thus it takes less timing to light it off.

My car, 1991 stopped going. Like yours the oil pressure goes to max vice 0 at start. I traced it to a bad oil press sensor AND wiring harness.

My ignition module tested fine though, but I'm glad it worked for you.

 

If one cat was empty where did the catalyst go? It doesnt just vanish, are you sure chunks of it didn't just travel downstream and plug somewhere?

seems kinda odd only 1 header glows but I don't know what your exhaust is like but if like my 90 it would be 2 into one into two.

did you punch the cat that was empty?

oops misread both your headers glow, what are the chances the remains of the hollow precat went downstream to the main cat?

 

No chance. I ran a snake through the pipes, cut the mufflers off (they're still off) and ran a leaf blower through both pipes for sport. I don't have pre/main cats anymore anyway. I am running a dual exhaust setup with 2 random tech cats (now hollow).

There's no restrictions.

Didn't get a chance to touch it this weekend, went to Colorado with my wife, so will get back to it with a new chip on this coming Saturday with really low timing in the tables. Hopefully that will be the cure.

 

maybe get some AvGas or other super hi octane fuel and run the vehicle on that fuel and re-check for knock/retard. if it still persists, then I'd guess the problem be in the knock sensor circuit. If it goes away, that'd be another data point. My guess is that your knock sensor is picking up valve train noise in the mid rpm's and retarding timing severely in that range, causing the headers to glow red.