Vehicle: 1988 IROC tpi L98; 150k quite gentle miles.

Issue: Runs sweet and sounds strong until RPM's climb over 4000; plus there's a big hesitation right out of the box for a split second. My (for those of you familiar with this device) G-Tech gives a very low 'net rear-wheel' horsepower figure (around 115-135hp); yet compression ratio's are excellent across the board. It is starting to get some blow-by the valve guide seals.

I've been going through and updating lots of little fubar's over the past few months. I did a full tune up with new fuel filter, added a AFPR and set the pressure at 37psi. Installed the twin cotton air filters, cleaned both the plenum and throttle body, added Taylor 8.5 wires, cap/rotor, Denso Iridium plugs; verified the distributor power thingy is putting out correctly(unlike a woman I know). I fed it with REDLINE 5-30 weight oil, which incidently really smoothed out the idle, unplugged the 9th cold start injector and always run premium fuel. I had the 700R4 tranny completely flushed and checked along with the TV cable adjusted. There is a complete 3" exhaust.

When accelerating or just revving up the engine from idle, she sounds VERY healthy till the RPM's start climbing up near 4grand and then it's almost like the engine power band has Adult ADD. It's not a miss, it's just kinda like nothing is working in harmony. There's suddenly some engine noise and it sounds like it's loosing it's breath and/or focus. I've been running Seafoam fuel cleaner for the past 6 tanks of gas. There are no codes thrown and the high flow cat is new.

Is there any area of focus any of you can think of that I should start looking at? I'm really open to suggestions.
Nitro

 

cam?????

 

Ignition timing?

Have ya pulled your plugs after install...if so, how do they look? Any odd-*****?

Any issues under load, or only at higher rpms?

How's your distributor? I've seen cars get funky issues at higher rpms and on one in particular, I found the rotor fit loose on the shaft, allowing it to wobble around, and it was scattering the spark.

Also, TPI motors are not known to be high-revvers, nor make power up there....they're torque monsters. Are you sure your tach is accurate?

 

Everything other than the few items I've tinkered with are stock, including the cam. Why would the cam make it run rough? I would think that the cam on this type of engine design would run smooth and steady even if it did run out of breath around 5k rpm??

That is a GOOD question on the rotor, as initially I was having a difficult time getting it to run correct and found that it wasn't seated correctly! The AC delco plugs I pulled out were worn out so badly that I was surprised there wasn't a misfire when I got it. They were the perfect color though, so I havn't gone so far as to pull the new ones out ( I just had major spine surgery in March and squating down still freaks my body out).

No, I don't know if the tach is correct, although the 700R4 seems to shift near the correct rpm times the repair manual states.

How does one tell if the distributer is acting up/out? I had my mechanic verify that the power module was good- but I am completely unfamiliar with GM distributors so don't know if under load what items will be stressed. I did pick up a billet ACCEL unit that's on my parts table waiting to be installed.

 

I missed that question.

it dosn't seem to matter if it's under load of just driveway throttling. It was running extremely rich when I got it and the prior owner had just replaced the factory catalytic converter. It was the 9th injector/Cold Start system that was on it's way out. I put another 1000 miles on it before I was able to revamp the fuel system. I'm hoping that the new cat wasn't damaged at all during that time. The fuel filter was so plugged that I could barely blow through it when I pulled it.

Is it possible that the valve train could be weak or worn enough that I'm experiencing valve float? Or if the valve seals are worn enough to cause any unwanted valve train movement? There is engine noise at that mid to high rpm range and I havn't been able to ID where it's coming from yet....
Nitro

 

......Anyone else have any ideas????

 

what kind of sound does it make at 4000RPM up? light rattle? loud clunking? ticking? Can you describe it?

Only under load, or if revved in the driveway too?

Like nixon mentioned, maybe your distributor has too much end play in the shaft. If you have a new one on the table, maybe try swapping that in. Check the gear endplay while it's out though, some new ones can be out of spec.

I don't think it's valve train, but you can check that. Pull both valve covers, and start the motor, and just look around... I'm guessing that's ok, as a problem there would be in the full RPM band.

My sneaking suspicion is spark. I think your new distributor will probably help out.

oh yea, iridium plugs aren't necessary, and probably aren't helping any in that motor.

 

First thing I'd try is a different distributor. Even worn shaft bushings can cause weird problems. Mechanical faults won't set any codes.

I can't see it being a valve train problem if it's only acting up above 4000 rpm. There's a possibility of weak valve springs and the valves are starting to float at 4000 rpm. typical SBC factory type valve springs should have about 80 pounds seat pressure. It should take a little bit of effort to push the valve down with your thumb. Without an on head valve spring tester, you can only guess if they're weak. With the "out of breath" complaint, this may be a possible problem. Valve float isn't the lifter sailing off the top of the cam lobe. Spring pressure when the valve is open is more than enough to keep the lifter on the cam. Valve float is when the spring slams the valve closed as it's coming off the cam lobe. If the spring isn't strong enough to keep the valve closed, the valve will bounce on the seat. This isn't good because it's allowing cylinder pressures to escape which will act like a loss of power. Even a broken valve spring can cause the same problems.

I'm also thinking of an engine imbalance but something like that should be noticable even at lower rpms. If all the internals are still stock, there shouldn't be a problem. The only thing external that could cause that is something like a mechanical engine fan, vibration dampener, flywheel/flexplate, clutch/torque converter. However if any of these became damaged enough to cause an imbalance, it should be noticable even at low rpms.

Does it vibrate while in park at 4000+ rpm? That would isolate if it's a driveline vibration.

A scan tool that can measure spark energy to each cylinder would be good. It could help to isolate a single cylinder or if the miss is in all the cylinders. One bad spark plug or wire may not show up unless the engine is under load. Put a timing light on a plug wire and rev up the engine until it misses. Does the light stop flashing. Do all the plug wires. Cheap way to see if the plugs are firing. Plugs don't fire, timing light doesn't flash.

You've changed your plug wires. Was the miss before or after this change? Did you mix up the firing order. Mixing up cylinder 5 and 7 wires is a common thing to do because they're right beside each other on the cap and on the engine. The engine will still run but will lose a lot of power because the firing order on those 2 cylinders isn't correct.

 

The car ran sour but passed California Emissions test when my wife brought it home to me. It ran like it was very tired. It definitely had higher RPM issues and didn't seem to know when or where it wanted to shift, but there was no spark 'miss'.
I was going to measure the horsepower/torque along the way with my G-Tech but there was just so many compiled issues that when I touched one thing it led to another and another; from the clogged fuel filter to the dirty phlenum. Surprisingly, the old spark plugs, although completely worn out were the perfect tan color with no hint of oil or fuel, even though the 9th injector was freezing open half the time.

` The noise IS like a rattle; definitley no lower end clunking.

` I will do the timing light check on Monday. Any ideas why it ran way-way smoother when I changed the oil out to Red Line 5-30? I havn't pulled the first oil filter and cut it open yet; maybe I should do that also on Monday (Monday is my personal car day). I've been taking a different car out to my local drag strip on Wednesday evenings as I keep finding things that need replacing on the Camaro so don't have any official 1/4 mile runs yet.

` On these pushrod engines, I can pop off the valve covers and oil won't fly everywhere???

` When I start the car it sounds wonderful (now). If I tweak the throttle I get a nice smooth healthy roar out the 3 inch exhaust- very clean and crisp. If I continue to slowly roll up the throttle it gives a deepening throaty roar and THEN, right as I'm nearing the 4k band it starts to fall apart. Then the rattling starts to settle in.

` Hopefully I can run a SEARCH and find out how to check the replacement distributorend play. It came off a wrecked car and had less than 1000 miles on its rebuild.

~ Denso Iridium plugs are supposed to be overkill on our L98's, but I've had them clean up engines that were using a bit of oil or extra rich fuel flow. I wanted to have as safe an ignition as possible while I'm sorting out all the continuing issues.
Thank You everyone for helping me focus!!!
Nitro

 

With the rocker covers off and the engine running, it may throw some oil. Mostly it will drain down off the heads and onto the exhaust. Try not to do it with a real hot engine. Some spray shields made from cardboard will help keep the oil in the head.

So you're getting the rattling as you roll up on the throttle while in park. As mentioned somewhere up above, with the rocker covers off and the engine running, watch all the rockers. They should all be moving up and down the same distance. If you have a worn cam lobe or a collapsed lifter, one rocker won't be moving very much.

I'm still going with a valve spring problem but without actually being able to personally troubleshoot the fault, it's just a guess.

Cut the oil filter open before doing anything. Take a section of pleats out and squish them in a vice to get most of the oil out. Then open them up for inspection. Any metal in the filter will be easier to see. If the filter is clean, then start tearing into the engine to find some other fault.

 

I went out this early evening and did several runs on my G-Tech, which for those unaware, the G-Tech measures 'NET' horsepower, which is real world horsepower taking into account ambient temperatures, wind resistance, etc. It's definitely a lower value than measuring the rear wheel 'dyno power' figures.
I made 5 seperate runs and what averages out is these figures:

Vehicle weight with driver and full tank of fuel= 3720 pounds
H/P= 130.1 @ 4199 rpm
T/Q= 168.0 @ 3850 rpm

I have always opened up the first two oil filters when purchasing a used car to see what's going on; and then about once a year after that. I'll definitely do that this week.
Nitro

 

Ignore the Gtech.....it's worthless as a diagnostic tool.

Try a distributor first if you can get your hands on one.... I'm wondering about the timing chain too, maybe it's real slack, enough that it's whacking the cam timing out a bit. But normally a worn chain will show up more in the lower rpm band, as the rpms tend to tighten it up..... Dunno

 

....I got extra busy this past week and havn't put any extra time into resolving what's going on. I did get a PROM made and I did install, by: PCMforLess.com.
It did take care of the tip in throttle issue, and I have great consistent torque all the way up the power band. I can still hear the engine rattle noise, but only had time to drive the beast once.

I would think that the new PROM chip is making up for either fuel or timing? So the issue is just disguised?

I also tried doing a SEARCH to find out if there is a 'better' knock sensor than the factory part. It's to bad the LT4 system won't work, and I did have PCM dial down the sensitivity of the knock sensor. I guess I'll just head over to my local NAPA Auto parts store and see what they have. I'm also going to have a friend supervise me while I switch out the factory high mileage distributor for the Accel unit, and hook up the MSD6. I also picked up a doctor's stethescope so I can hopefully narrow down the engine noise.

I really want to express my sincere thanks for all of you walking me through this tricky diagnosis issue; Call me ****, but I want it to run as sweet as it can. I learned so much from this post set!
Nitro

 

Did you ever find out what the problem was? My '91 Trans Am is doing the exact same thing, its driving me nuts.

 

I've been inundated with busy work and have not done further checking since I installed the new PROM- That item is by far the very best investment I've made yet on this car.
I'm having my suspicions that it might be related to my stock injectors; it's not ever a matter of those going bad, but more of 'when'. I bought some SVO 24 pounders and just got them back from being cleaned and new seals installed and will put them in next week.
Have you run your timing light off of each of the spark plug wires to check for individual cylinder spark issues? That's by far the very first thing you should check before any other step.
I did take the car out yesterday and did several G-Tech runs: they all came out very-very consistant, with reading up just over the 20 real world net horsepower mark for increased ponies. Are you still running the stock PROM?
Nitro
Nitro

 

Stock Prom as far as i know, i just bought the car, i havent done a timing light off each plug yet, i plan on toying with it tomorrow weather permiting, there is a remanufactured distributor on the motor, i was hoping it was that as i also have a new distributor on the parts counter. tomorrow i hope to get some wrenching in, and will definatly check each plug.

 

I'm busy-busy at the moment, but I'll let you know in detail what all I've found, what works, and etc.... It'll be a day or so though, okay?
Nitro

 

Okay, I'm finally able to sit down for a few minutes; I just had my Spohn subframe connectors installed and an exhaust issue redesigned.

You havn't presented much info on your new ride, so I don't know how much miles have built up on the factory parts or if you know how much mechanical care it's had, so I'm shooting in the dark here.
Here's the issues I found on mine, although the initial worst of the worst
culprit ended up being the 9th cold start injector which you don't have on yours (it was deleted by GM in 89).

The thing about running a timing light off of each of your eight ignition wires is to help diagnose if the issue is spark or fuel related, which are two of the most usual suspects and pretty much need to be near 99% perfect before going on to checking sensors.
Do you have any check engine lights showing; and did you do a self check to see if there are any codes that have been thrown? Start there first.

I had no codes thrown, so I pretty much could rule out any bad sensors. depending how much R&R you've done in the past; I had a gut feeling that mine was mostly fuel related, so I checked first the fuel filter. Sure enough, it was so clogged I could just blow thru it.

Oh, just for reference, I always count on spending an initial $1500 on a used car when I purchase it- that's what they almost always need to make them roadworthy. If I don't have proof of any prior mantainance, I always start from scratch and do a full-full vehicle tune up, that way I always have a base line to reference from should it not be running right. I also do some quick research to find out what parts are better than others. I absolutely do not use any Kragen/Autozone/Grand Auto type parts unless they are carrying the higher end parts.

After replacing the fuel filter it still ran like crap, but was there was a noticable smoothing of the idle. The same happened when I changed the oil to RedLine 5-30; smoother yet.

I then replaced the ignition wires, which were cheapo wires that were small, aluminumed end, and brittle. I went overboard and installed Denso Iridium plugs. They arn't necessary, but they will clean up a cylinder that's dealing with a bit of blow by (aka: worn high mileage cylinders). You might take the time to read their Denso info site, it's really packed with good info.

After I did the complete ignition tune up the car still ran like a weak turd- no noticable difference, although my mileage/fuel went from 9mpg to 12mpg.

Then I pulled my 9th injector and it stopped stalling at intersections. But again it still ran rough, just a much more consistant idle.

After all that, I pulled the plenum so I could clean all the built up grunge from the throttle body on back, and there's usually quite a bit. I then installed an adjustable fuel regulator and put it all back together with the new throttle body air foil. What I really-really should have done, and it was totally stupid not to, was to have budgeted for having the fuel injectors cleaned and checked for less than a 2% flow difference. (average price is around $25 each injector)

Each year GM would change the injector manufacturer, and it is my understanding that around 1990-91, the worst brand of them all was used. All of these injectors have a short life span, especially compared to japanese products, thus it's a matter of not 'if', but WHEN they need to be pulled and cleaned. I did a big SEARCH in the injectors and found out the hands down favorite is the SVO (Ford) 24# injectors are one of the best. STAY AWAY FROM ACCEL INJECTORS!....
I think they cost me about $95 bucks and were low mileage units. I still went ahead and had them flowed, as I really want this F-Body to run at peak performance. It turned out that they were almost perfect in their used state, well within the 2% tollerance. Now I have to pull the fuel rails and crap all off again to access the old injectors; should have waited, but I was tired of chasing the crappy running issue.

At this point, It started running much-much smoother but now I was at the point where I started this post. I also sprayed the whole top of the engine with carb cleaner to check for any vaccum leaks and I had the distributor ignition module checked and the throttle body adjusted. Your ignition module if it's starting to die will run great at low RPM, but under load will start to peter-out.

Have you already purchased a copy of: CAMARO PERFORMANCE HANDBOOK by David Shelby??? That will give you the info for tweaking the throttle body and many-many other Fbody related info.

I know I still have an issue that the new PROMforless.com 'chip' has covered up, but with the new PROM it runs fantastic! I did my G-Tech runs and suddenly all my acceleration runs were even and strong. Before that, every run was within 50+ horsepower of each other; scarry! I definitely got a solid 20 horsepower increase, and the power comes on from about 2500rpm all the way up to almost redline.

I still need to change out the injectors, the distributor, and check a couple other items to know where I'm currently at. Plus all my neighbors are much happier not hearing this loud loaping engine that sounds like a worn out 1970's big block pickup truck (full 3" exhaust with flowmaster).

I hope this helps! Please let me know what you find, seriously. The more information we can add here will help the next frustrated person, Okay?
Best of Luck to Ya,
Nitro

 

For some back ground on my car, the car itself has 144,000 miles however the previous owner had the engine rebuilt about 6,000 miles ago. I did notice that there is a distributor from AutoZone in the car while i was swapping out the cap and rotor. i changed the plugs and fuel filter as well. Definate improvments with both idle and fuel mileage but still looses it at 4k. Its been ungodly warm where i live lately so i have been unable to swap out the distributor. Most likely i will be looking at doing the project this weekend as it will be about 20 degrees cooler. I will let you know how it goes.

 

Bad News, The distributor swap didn't solve the 4,000 RPM stall. However it did eliminate the first mini hesitation. I installed the distributor at the same timing point as the old unit. so i will be checking the timing tomorrow. The battle rages on.

 

What was the timing set to?

 

I may have the solution. I talked to a fellow co-worker today consurning the problem. Apparently he had the same exact issue with a carburated car he had in the past. I guess a good valve job is in order. He didnt think it would be the heads because his car still had such good vacuum. But sure enoug he swapped out the heads and never delt with the 4000rpm cap ever again. I know my heads do have a good 150,000 miles on them, so i figure its about time. plus what better time is there to sneak in a few mods... headers lol

 

I know this was posted 9 years ago but was the problem fixed by a new valve job. my girlfriend has a 83 Z28 with a 305V8 and a 5sp. i have all new plugs, wires, cap, rotor, ignition module, new vac lines. when it hits 4000 RPM's it's like it's starving for air and wants to die on you. runs great up to that point. motor has 159,000 on it. motor is bone stock.