So I've had My 94 LT1 for 22 years in october, It's been running like crap for years, I just don't drive it, it sits in garage, as I have not had time, or just been lazy, now I gotta get it going. Now I need like actual expert help, I've worked on hundreds of C4, I figure out what shops can't on more than one occasion, I've parted dozens out, know every nut bolt, and size by memory on 98% of the car, While people talk of 8 hour heater core jobs, I can do it in 3 hours.

Currently car starts right up idles for a second, struggles and dies, If I restart and feather the gas it'll stay running, but like crap, and backfire through intake. Now before ya'll start throwing optispark, did you wash the engine? change the fuel filter, here is what all I have done, since it started being a buttcheez. I replaced the opit twice, basically no miles on current (or water), changed the ignition module, the coil, the plugs twice (second set today when I found 7 wasn't firing, but inline plug tester did.) I took out the less than 500 mile 28lb injectors and put in new factory flow injectors 0 mile. New 8mm plug wires, all of which light up my inductive timing light on each wire, Rechecked the OPTI firing order 3 times, cleane
d the MAF, I switched out my ecm with a 94 I was parting no change, and that car ran good. I pulled the valve cover to make sure the roller rockers didn't loosen, nope, no bent or broken rods all rockers rock, compression test good, fuel pressure over 42#, new gas. YESTERDAY after feathering and letting it warm up, I could smell hot metal, the left pipe AFTER the cat was red, I pulled the O2 sensor out before the cat, still starts shuts off. The ol remove plug wires/ disconnect injectors till it doesn't change won't work as it won't stay running long enough. No useful codes, Car has 98k I'm about ready to take it to a shop, and know it's gonna be 'Well it's the optispark, and we'd like to change the fuel filter, and it will be the same, and they will have their hand out. Pic from when It ran good

 

"no useful codes" isn't gonna cut it for this.... We need to how you are pulling codes, do you have a scanner? If so, need every code in every module including Current or History. If you are doing paperclip method,. check the ECU water temp sensor vs one that reads to dash. Also interested in IAC and TPS readings. Fuel pump on start you mentions is #42, did you keep a gauge hooked up and see fuel PSI while running and also measure leakdown time? Probable going to say pull the fuel rail and do a test with injectors mounted on rail and pressurized.

 

The hot cat symptom could indicate that there is extra "un-metered" air entering the system. I used to have an 95 LT1 ImpalaSS and the common cause was a broken exhaust manifold bolt (usually a rear bolt would break). This would allow a little air to enter the exhaust system, the O2 sensor detects this oxygen and this tells the ECM to give that engine bank more fuel. But there really isn't extra oxygen waiting to burn more fuel, so the extra fuel does not get burnt and it hits the hot cats...burns and gets the cats even hotter. Find the source of un-measured air getting into the system fast or that cat will clog. Once this is resolved, you may want to replace the cat (use your judegement). What i mean by un-metered or un-measured air is any air that is getting introduced to the engine after the MAF, i.e., it's air that the MAF doesn't know about. Check for vacuum leaks, gasket leaks, exhaust manifold leaks, manifold to pipe leaks, etc. Wouldn't hurt to replace O2 sensors, as well.

Starting and then immediately dying...my first check is always for a vacuum leak. Once I was replacing the EGR and forgot to put the brake booster vacuum line back onto the intake (I had removed the line to get a little more room to the EGR). The next thing is to check fuel pressure...you did and it seems good. Then ensure the TB is clean. Then there's the Idle Air Control in the bottom of the TB...I replaced that once on the old Impala.

You mentioned plug wires...probably not related, and I know you said you checked, but the more than one LT1 owner (myself included) have mixed up the plug wires on 4 and 6. It is easy to get those two crossed...it will run at low RPM but will let you know it isn't happy fairly quickly in the RPM's.

You may do some searching over in https://www.impalassforum.com/ as there are a lot of LT1 operators there. Best of luck!

 

Could be restricted cat conv. Pull O2s see it it makes a difference. Red exhaust pipe isn't good, Overfueling or restriction

 

Did you take out both O2 sensors? I would loosen up and drop the front exhaust pipes to see if it will run any better.

 

Long shot-But I would check the FPR. When mine went bad it had similar symptoms as yours including glowing converter. Pull the vacuum line and see if it smells of fuel. Mine had fuel in it.

Good luck,
Steve

 

I would lean towards the plugged catalytic converter as that makes them get HOT. Pull the O2's and see how much back pressure is on that side. If more than ~5 psi then you have an issue with your catalytic converters. Anything Glowing red will set your beautiful Corvette ablaze if not taken care of.

The FPR is an easy test on an L98 but I have no idea on a LT1 engine. On our L98's you simply pull the vacuum hose off and smell for raw gasoline. If the fumes are present then you need to replace the diaphragm in the Fuel pressure Regulator. This is very common as the diaphragms were not made for use with ethanol, the ethanol will cause them to break down quicker than normal.

I love the color of your 1994! That yellow is stunning on a Corvette.

One thing that has helped my Corvettes and other vehicles idle smoother is to run a can of B&G 44K Fuel System Cleaner as it does a really good chemical cleaning on the parts. I suggest it whenever I hear of rough idles. I give you my "Word as a Gentleman" that it is not a Snake Oil as some so dearly call everything sold in the aftermarket.

Your MAF/MAP could also be a part of the problem you are having. The MAF/MAP measures the precise volume of air being ingested by the combustion process. If there is a leak or the sensor is acting up the car will have issues revving smoothly. I would think you would have a MAP sensor not a MAF sensor like used on my 1988 L98 equipped C4. If you do actually have a Mass Air Flow sensor cleaning it doesn't always work. You need to be extremely careful as the wires are roughly 1/10th of the diameter of a strand of hair and can easily be broken. They make a spray for cleaning the MAF sensors but the very best thing for it is to be cleaned by the "Burn-Off" function. If you really have a MAF be sure that both relays are working properly for the MAF. When the relay fails and the burn off function can't take place problems arise.

If you have a MAP sensor be sure that there is no air leaking into your system around the sensor or the throttle body. Any extra air leaking in will cause errors in the MAP or MAF sensors. On my C4 with the L98 I have to check the tube that goes from the MAF to the throttle body to ensure that there are NO air leaks or the car will run poorly and rev poorly.

 

A plugged cat isn’t going to keep the car from idling. It will cause an issue driving under load and will limit performance.


I’m going to hone in on fuel pressure. It sounds like you are getting pressure at key on, but it doesn’t maintain pressure.

 

Ive pulled the O2 sensor up stream of the cat, still sucks. There is no fuel in the pressure regulator vacuum line. I have the OBD1 plugs that fit the OBD2 shap like a 94 and 95 have, is that useful? the car has pressure at start and run. The injectors are new, and it's a second set it holds pressure.Everything suggested, I've already done, and pulling the cats o2 to relieve pressure to run was the last thing I tried, I know they'll generally idle.

 

While not on a Y-body, last time I had a glowing red catalytic converter was when half the fuel was not being ignited due to a bad ignition component and the raw fuel was fed through the cat, just saying.

 

If you don't mind buying something, I'm going to throw my hat into the ring and suggest you purchase an ALDL cable for a computer. Like this one: http://aldlcable.com/products/aldlobd2u.asp

That way you can plug a computer in and monitor the actual sensor data from the PCM in realtime. Even more importantly you can log that data and send it to us fine folks here on the interwebs so that you can have even more eyes on it, see if there's something super obvious going on.

Just for absolute ***** and giggles, have you tried unplugging the MAF sensor? Doing so will flip the PCM into speed density mode like the 92-93 (and throw an error code, but we can ignore it). Just curious if that'd change anything.

Also I'm sad you're in Florida. I'd gladly pay you to replace the heater core in my '95, but you're on the other side of the country.

 

My L98 had very similar symptoms with plugged cat. Though wondering why a new plug (#7) wasn't firing - have you checked ignition wiring and connectors, any loose or discolored pins etc?. Datalogging would be beneficial, so many parts already replaced. Was the temperature sensor also changed or checked?

 

I do a lot of work with my son on old cars that he buys to restore.. a favorite of his is "crank no start". There are a bunch of tricks we learned to figure out roughly where the problem is. For example - fuel - disconnect the feed lines and point it at a container.. and switch the ignition or crank.. make sure there is a decent amount of fuel coming out. MAF - disconnect it.. most cars will run in speed density mode without a MAF and it will help isolate the MAF. Disconnect the exhaust at the headers - a clogged exhaust is very common (collapsed Cats). Ignition timing - make sure #1 is firing close to TDC when cranking. In your case the red hot exhaust after the cat is important.. . Cats should increase the exhaust temperature by about 50-100 degree - use a temperature gun before and after the cat - if it's completely clogged you'll just get a temperature slightly lower than the incoming temperature. My thought is that the cat is not clogged but (per others here) is dealing with excess fuel.

Since you have new injectors, and a new ECU, either the timing is way off or one of the sensors is driving the wrong values either because it's bad or it's not reading everything. Unmetered air would cause it to run lean, so maybe not the problem, but still worth spraying some starting fluid around the intake and vacuum lines to see if the rpm increases. Are the plugs actually firing (i.e. is there a clean path to ground from the engine?). Does the car think it's minus 40 degrees outside and adding a bunch of extra fuel? (air temperature sensor).

Personally I'd get a scan tool and eyeball all the sensors to see what's out of whack.

Good luck!

 

..another thought - my buddy had a 94 and it was extremely easy to get the opti off by one tooth (he did it twice when going through about 5 ebay optis in 2 years). If the scan tool shows the sensors are ok, I'd get down to the opti again and tweak it one tooth (hard to measure, I know). You can run the car for a minute without the water pump so it's easy to test before putting everything back on.

 

I mean, it's keyed

 

correct, but like I say my buddy managed to get his installed wrong on two separate occasions. I think this is why later years have a pin rather than a doubled tooth setup. Or maybe cheap opti’s have softer material. Either way- check your timing.

 

You mention that you have roller rockers and were running 28lb injectors.
What other mods have been done to the engine?

 

Car ran fine after that, and now have factory 22lb

 

I've seen folks suggesting unmetered air leaks and bad grounds in this thread. I was going to suggest the unmetered air myself, but the bad ground is also something worth looking for.

 

You have a lot of suggestions, so I am not going to type a lot here... have you unplugged the MAF?

I have had two red exhaust pipes in my day. 5.0 Mustang was clogged and an H-Pipe with no cat fixed it

My L98 had a bad MAF causing stupid rich condition. Converting to speed density fixed that.