LT1 Diagnostics

Okay, first the disclaimer... I haven't had every one of these problems, nor diagnosed them all myself. I also can't guarantee that if the problem you have looks like the one I describe, my answer must be correct. I've just collected this list, some from my experience, some from hearing about other people's problems. Hopefully it'll be useful. For those of you that have contributed, or see your writings in here - thanks very much! As usual, if you see inaccuracies in what I've written, please email me, I'll be glad to change them.

Also, if you're ever wondering what or where a part on your car is, or need a diagram of something, check out shoebox's 4th Gen LT1 F-Body Technical Aides page.

Many ignition problems can be realized as high rpm roughness. Check for intake leaks around the MAF and throttle body connections, tighten your plug and coil wires and use dielectric grease for better contact, maybe replace plugs and wires, but there's certainly a chance it's the the Optispark. If the problem gets worse as the engine warms up, to the point where the engine dies and won't restart, you might have a bad coil. In order of what's easiest to do, replace the coil first, then plugs (you might try gapping them down to 0.040" rather than the stock 0.055) & wires (including the coil wire). But if the problem remains after all this, you probably have a bad Opti.

A hesitation at high RPM, if it's not an ignition problem, can be a lack of fuel. Are your injectors at 100% duty cycle? Check the fuel filter, and the pump.

Wheels: Shake it top-to-bottom to check for a bad wheel bearing. Shake it side-to-side to check for a tie rod.

Low RPM vibration, especially in high gears: Perhaps the easiest resolution here is a loose or burned spark plug wire, or even a cracked plug. If you've just installed headers, it's almost guaranteed that you've got this issue. And it's also likely that it's wire #8. Check by looking in the engine bay at night, looking for stray sparks. The plug could be disconnected at the Opti, or the plug end, or burned in the middle. If you recently did headers, you may have also crossed some plugged wires. You can also test to see if any cylinder is firing by dripping water on each primary. On a hot engine, each should sizzle, unless there's no fire in there. The other way you may not get a cylinder firing is lack of fuel. Make sure the injector is plugged in, check injector harness.

Headers red hot: Rich fuel condition - fuel burning in your primaries. Can be caused by un-metered air being pulled in downstream of your MAF, dramatically retarded timing, low spark intensity (incomplete fuel burn), possibly a bad optispark, even a cam installed wrong.

Cats red hot: Rich fuel condition (fuel burning in cat). A rich fuel mixture can be caused by a header/exhaust leak sucking in fresh air, causing the O2 sensors to read a lean condition and adding fuel.

Gear Oil Leak: Oil leak from front of axle pinion seal – could be overfilled differential case - check it while the car is on a level surface. If that's not it, you need to replace the pinion seal.

Engine Oil Leak: The LT1 motor is known to have a badly sealed intake manifold. Reach around (from the top) behind the intake manifold, and find the joint where it sits on the block. This is where it leaks. If you have oil leaking up here, you'll have it on your hands as you feel around. The fix is to remove the intake, and re-install it with new gaskets and gasket sealer, letting it dry overnight before starting the car up.

Low Oil Pressure at ½-WOT: It isn’t your filter guys. Get a mechanical oil gauge and check if it really is happening. There is a place to hook it up just above where the filter screws on to the block. If pressure drops, then you need to check you oil pickup tube height, your pump and lastly change you bearings. A few have said to use heavier oil. This may mask the problem, but it will rear its head again.

Oxygen Sensors: At wide open throttle when the PCM is running in open loop, O2 sensor values should be around 880-900mv. In closed loop operation, it’s constantly richening/leaning things out trying to get to stoichometric, ~450mv, so you'll see readings jumping up and down hundreds of millivolts, centered around 450mv. A "dead" O2 sensor will read around 450mv all the time, and needs to be replaced.

If you're running really rich, you can even get flames out your exhaust. Especially if you aren't using catalytic converters. The most likely suspect is a bad O2 sensor (check them with a scan tool), or maybe a leak in your exhaust system in front of a good O2 sensor.

A hesitation or stutter at low RPMs can be many things. Try replacing your EGR valve first, then O2 sensors.

A hesitation or bog might be caused by a leak in your air intake. Check all connections from your filter all the way to your throttle body (throttle body elbow, MAF, etc).

Car not starting: The key turns, but all you hear is clicking? I assume that means the engine is _not_ turning over via the starter? If that’s the case, it can’t be your coil or ignition. It’s either going to be your battery (are you certain it’s fine?) or your starter/solenoid or your engine is seized up due to water in a cylinder, etc. If it’s clicking then I already ruled out a PASS key foul up. If the PASS key security feature locks you out, there’s no noise at all. If your starter won’t even engage, after ruling out a weak battery, check the gauge fuse in the fuse compartment on the end of the dashboard. Another possibility: if you know the battery is good, but no lights come on in the dash gauge and fuel pump doesn't run, you may have an ignition switch failure in the switch at the base of the steering column.

Tranny low rattling sound when in neutral: In my experience, pilot bearing. Small cheap part, never reuse in clutch re-do. Most T56 transmissions rattle like this, don't worry about it until you need a clutch.

Coolant light on: First double-check you actually don't just have low coolant, and bleed the system. The coolant level sensor (right at the neck of the radiator) also gets quite a build-up of crud, so it can be fouled.

Coolant temperature rising: If there’s plenty of coolant, check for air in the system by bleeding it. Make sure both fans run, and that the thermostat isn't broken, and that the timing isn't too retarded or the mix too lean, and that the a/c condenser is not plugged up with leaves, bugs, etc.

SES Light: Be sure to check all the fuses and do a scan of connectors under the hood first. Once you've done that, you'd better use a scanner to find out what the computer's telling you.

SES Light for fuel: PCM measures pressure in the fuel tank, should be around 2psi. It also measures on startup, to verify tank doesn’t lose pressure over time. It only checks, though, if the fuel sending unit in tank registers between 20 and 80% full tank.

The gas gauge on f-bodies is notoriously bad - don't even bother trying to get it fixed. By the time it gets down to about halfway, you've actually got about 1/4-1/3 of a tank left. You just need to get used to that.

Knock: One way to determine real knock versus false knock is to run the engine on high-octane gas, like 100-octane unleaded (not mixed with 92 - get as close to 100 as possible). If the knock goes away, it was real. If it’s still there, it’s false. By the way, you can't use octane boost for this experiment. "8 point boost" will get you from 92 octane up to 92.8 octane. Another alternative to finding high-octane unleaded is to make your own, using toluene or xylene. They both have about 115 rm/2 octane, and in a 30% solution with 92 octane gasoline either will get you very close to 100 octane. See http://www.gnttype.org/techarea/misc/octaneexplained.html for a detailed explanation.

Exhaust Colors: A small puff of white smoke on start up is considered to be normal on a modern emissions controlled vehicle. If the smoke is occurring at other times that start up, something is wrong.

Black Smoke – Rich condition. Usually visible during acceleration but can occur at any time. Excessive fuel, could be caused by a zillion things; MAF, TPS, leaky injector, bad O2 sensor, etc…
Blue smoke during acceleration – Oil leakage past rings. Confirm by measuring compression, squirting oil in spark plug hole and re-measuring. Change should be less than 10%.
Blue smoke during deceleration or on start up – Bad valve stem seals (valve guides).
White smoke acceleration – Coolant leak into cylinders. Possible causes are leaking head gasket, intake manifold or TB coolant leak. On the LT1, the intake manifold does not carry coolant, so you can rule this out. Frothy coolant? Head gasket, for sure!
White smoke during idle or deceleration – leaking intake manifold gasket. Not likely on LT1, see #4. Possibly – extreme case of blown head gasket.
Smoke under deceleration means the valve guide seals are letting some oil go past them.

Squeak from rear on sharp turns: If it makes the scraping/screeching sound on hard turns, this is your most likely problem. It's just a matter of shimming your caliper out, that’s all, and would take about 30 minutes to repair. The sound you're hearing would be the caliper itself hitting the rotor on the turns. One quick test for you to do is to go into a parking lot and go into the hard turn, once you hear the scrape, apply your brakes. If the sound stops, then its even more likely your caliper's hitting. What your doing is pulling the pads inward thus your calipers won't hit.

This is a fairly common problem with our cars since the specs are so small of a range...measurements are 68.625-69.79mm (2.70"-2.74") from the inside caliper mounting bracket to the outside face of where the stud bolts mount to. The shim btw is part # 26034667 If your rotor is out of spec on run out then your problem can be worse.

Gear Noise: Usually stock gears don't make noise, but if you have aftermarket ring and pinion gears, this noise is pretty common. More with some brands than others. There's not much you can do at this point - it's not dangerous, just annoying. It's unlikely that the gears can be adjusted to lessen the noise. The only answer is a new set.

Exhaust rattling: If you've got an aftermarket exhaust, that's the most likely rattle. Common locations include the tranny cross-member (exhaust pipe resting on it), or the upper Panhard rod, where the exhaust loops over the axle. For the tranny cross-member, a common fix is to drop the cross-member, and put a short stack of washers under it, to lower it down a bit. If that doesn't help, and the exhaust just sags back down on the cross-member, have an exhaust shop fab up a small bracket that will hold the exhaust to the floor pan right there. At the rear axle, you can usually adjust the height of the exhaust at the hanger at the rear.

Other loud noises can be loose suspension parts in the rear, including your Panhard rod or lower control arms. Check the bolts on them. Even louder noises can be caused by torque arms colliding with the floor pan, or sometimes your differential cover striking your upper Panhard rod during suspension compression.

Ticking noises: First possibility is exhaust leak; second is plug wire arcing or loose spark plug (very sharp tick-tick). Check those first. Then check for loose rockers (do the trick of putting a screwdriver on to your valve cover and the other end to your ear). After that, check oil pressure.

Clogged cat: High rpm power will drop off, sometimes low rpm power will seem fine. Exhaust sound may be more muffled. Header / manifold and cat may be very hot, sometimes glowing. Your temp gauge may read very hot and your coolant may boil over. If you have a pyrometer, you might measure the exhaust pipe temperature before and after the cat. If the cat is healthy, the temperature should be similar on both sides.

Car out of Level: Many folks have reported that their cars don't sit level to the ground, particularly as viewed from the back, or when looking at the rear tire gaps. First check that your springs are correctly installed, with the cut end at the bottom pointing toward the front of the car, and the top being seated correctly in the donut. Also make sure you've waited for new springs to settle for a couple months if you've just installed them. Some folks say that the cars ride a bit high on the driver's side, so that when someone's sitting in it, it'll be level - that may be, but on my car I'd have had to weigh about 500lbs to level the car out. Anyway, if you want a fix, I cut a piece of high density plastic, about 1/2" thick, in a donut to sit above one spring in my car, which has worked perfectly.

Driveshaft vibe: If you've got a stock steel driveshaft on the car, this is likely it. They were known to vibrate above 90mph, because of yokes being mounted a bit off-center. For the easiest fix, unbolt the rear of the driveshaft, rotate it 180 degrees, and bolt it back down. You may get lucky and counter-balance the offset weights. Failing that, either of the factory aluminum driveshafts (1LE, LS1) or a good aftermarket driveshaft should fix your problem, of choose a good aftermarket vendor if you're looking for an extra strong replacement. Of course vibration can also come from any other imbalance in the drivetrain or a unbalanced wheel, but the usual problem on f-bodies is the driveshaft.

Driveline vibe: Chasing down a driveline vibration can unfortunately become a full-time job. There's a whole section on it in the Factory Service Manual from Helms, if you're looking for professional help. To start with, add some logic to help narrow down the options. A vibration that tracks engine RPM's is going to be caused by something forward of the tranny input shaft. A vibration that tracks the car's speed is going to be something from the tranny backwards to the wheels. Options include driveshaft (of course), tires out of balance, bent wheels, a misfire, pinion out of balance, out of balance flywheel or flexplate, missing bolt in the flywheel, and probably more.

Tire wear: You can have unusual tire wear for a couple reasons. Most obvious is over or under inflation. If the tires are more worn in the center, they're over inflated. If they're worn on the edges, they're under inflated. You should check the pressure regularly (at least once a month), and if you don't know the correct pressure, check the label on the driver's door. If the front tires are wearing unusually on either the inside or outside edge, you may also have an alignment problem, and probably should have it checked.

Screech when tranny shifts: If you hear a screech when your automatic transmission shifts at full throttle, it's almost guaranteed to be your alternator belt slipping under high torque. You can even make this happen in a manual transmission, usually by being way too high in RPMs for the next gear. You'll probably need to replace it. I like the Goodyear "Gatorback" belts, they seem to have a bit more grip.

If you haven't changed your automatic transmission fluid lately, do it. Have a full flush performed, that will get the fluid out of the torque converter. Sometimes this can resolve a problem of the transmission sticking at the rev limit, and not shifting.

Your gas mileage can drop considerably if your oxygen sensors are going bad or if you have exhaust leaks in front of the sensors. Use a scanner to see if one or both sensors might be reading wrong (like a constant value around 0.450-0.500mV).

The ABS computer has been known to get wacky at times, causing a situation where you can be just using the brakes normally and the ABS kicks in, with the "low trac" light coming on. Try re-setting the computer by disconnecting the car battery for a few moments. If that doesn't work, pull all the electrical connections on your ABS unit, your wheel hubs and the rear differential case, clean and re-connect them. You may just have a bad contact. Last resort, you could actually have a bad sensor in the front hubs or the differential.

A common problem on f-bodies is a broken tranny mount. You'll probably find this after you hear a loud "THUMP" under the car every time you accelerate. It's an easy and cheap fix. The same problem could be generated from a worn engine mount.

The "ABS Inop" light is wired to the rear third brake light, so check to see if that brake light has burned out.

If you remove oil filler cap and see puffs of smoke coming out while the car is at idle, then you've lost a piston's ringland. This is unfortunately common for supercharged motors.

First, bleed the brakes. Air in the brake system will make for a very "soft" pedal with long travel. If you've thoroughly bled them and the pedal still pushes down to the floor, your master cylinder is most likely failed, letter fluid through.

Thanks to contributors: Jeff Stevens, George Marengo, Todd Klein, Randall Huls

Copyright © 1997-2004 David Mills, no part of this site (http://www.go-fast.org/) may be reproduced without permission of the author. The author makes no claims or guarantees as to the quality of the information on this site. I'm an enthusiast just like you, and while everything here is correct as I know it, I'm not responsible if your car breaks.

Suspension

Exhaust

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Driveline