Sick L98

58 Corvette with 1987 L98 engine, stock ECM, external Walbro fuel pump: Running great for months, then died one time 5 miles from home a while back, and immediately restarted - drove home with no problem. Then later, about 5 miles from home started coughing through the intake and sputtering (just like earlier) - eventually died after turning around to head back home. Waited 40 minutes restarted but hardly ran - called tow truck - embarrassing. Once home the car restarted - barely running but noted small amount of white smoke from left exhaust. Never any codes were thrown. Reset lifters from one turn past zero to 3/4 turn. Vacuum is good at 15 - 17". Checked compression and it was all around 120-130 PSI. #5 & 7 spark plugs are just a bit darker than all the others. Did coolant pressure check and it was good. Started and it was smooth and solid - went for test ride and performed well. Stopped at a neighbors about 300 yards from home - spent maybe 3 hours - went to drive back home and barely made it back. Engine would either run good for a short while or start sputtering and backfiring. Thankfully it ran strong for a few seconds to get me up my driveway and into the garage. Recently replaced everything in the distributor – no change. Runs terrible with the TPS connected or disconnected. I believe my grounds are good but I could be missing something there. Fuel pressure comes up to 45 PSI but will bleed down to 5 PSI within a few minutes. I then had someone turn on ignition in order to bring pressure up and I immediately shut the valve between the fuel tank and the pump. Same thing – pressure bled off in about 3 minutes to about 5 PSI then it takes a long time to go to zero. Any suggestions?

IMHO your problem is in the tank, either electrical or half plugged with gunk. If the tank is stock I would bet the outlet is partially plugged. I had a problem similar to what you are describing many years ago with a 57 Bel-Air 2dr hardtop and a contaminated tank was the problem.

I'd say Ken could well be right. Have you checked/changed your fuel filter? I remember my '71 Charger and a questionable Houston neighborhood one afternoon.......

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Originally Posted by NTFDAY

IMHO your problem is in the tank, either electrical or half plugged with gunk. If the tank is stock I would bet the outlet is partially plugged. I had a problem similar to what you are describing many years ago with a 57 Bel-Air 2dr hardtop and a contaminated tank was the problem.

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NTFDAY & RSPEARS I don't think that's the problem for the following reasons: Runs great every so often. I coated the interior of the tank a few years back and recently opened up the top and it looks clean inside. The filter between the tank and the fuel pump remains clean. I used to have a submersible pump and modified the top of my tank to accommodate the pump and fuel level sending unit so I have great visibility into the tank. Another fellow recommended I change out the CTS and I'm going to try that - right before I pull my hair out.

First I don't know what the "CTS" is, but because of your statement about the fuel pressure leakdown I'll toss out another thought. I was having intermittent starting issues with my '05 Jeep, and I changed just about every sensor it had - Crank Position, Cam Position, Throttle Position and finally took it to a buddy of mine who had a repair shop. After chasing things for most of a day he found that three of the six injectors were leaking down, bleeding off pressure and wouldn't hold against the pump head. I told him to change them all, and it was like a different engine. Wondering if perhaps you've got some bad injectors?

I had a similar problem with the Cobra some years ago. We too tried everything we could think of but to no avail.
It would run sweet as a nut for hundreds of miles, never missing a beat, then suddenly it would go ape.
In the finish, after twelve months or more of this, I pulled the fuel tank out and cleaned it thoroughly...and out came a waxed candy wrap paper that some little a$$wipe had dropped in.
No more problem after that.
We surmised it had blocked the outlet, when the motor stopped, so did the suction, and it drifted away.
Only to return at a later time/date.

So I feel for your frustration.
And the fuel tank could be a good place to look.

(And I put a filter on the filler-pipe to the tank.)

Have you checked the fuel pressure when it is running like garbage? That would help with diagnosing the issue. External pumps are prone to failure due to heat and lack of cooling. How far from the tank is your pump? Leaking/sticking/gummed up injectors could be it too. We had a 91 TPI Vette years ago that was running similar and we found after many hours working on it that the coolant temp sensor was shorted telling the computer the engine was over heating but the gauge showed normal. It ran all sorts of goofy. That car was probably the most finicky TPI car I was ever around.

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Originally Posted by rspears

First I don't know what the "CTS" is, but because of your statement about the fuel pressure leakdown I'll toss out another thought. I was having intermittent starting issues with my '05 Jeep, and I changed just about every sensor it had - Crank Position, Cam Position, Throttle Position and finally took it to a buddy of mine who had a repair shop. After chasing things for most of a day he found that three of the six injectors were leaking down, bleeding off pressure and wouldn't hold against the pump head. I told him to change them all, and it was like a different engine. Wondering if perhaps you've got some bad injectors?

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RSPEARS, CTS stands for Coolant Temperature Sensor, which I just finished installing and wouldn't you know I've got the same problem. Dog turd!!! I've got a new fuel filter which I'll stick in but I'd be real surprised if that was the cure. I guess my next question is: Since I obviously have fuel leaking somewhere it MUST be through one or more of the Bosch injectors - which aren't all that old. I have not found out how to troubleshoot which injector is leaking. Do you have any reference to how to trouble shoot that issue?

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Originally Posted by 40FordDeluxe

Have you checked the fuel pressure when it is running like garbage? That would help with diagnosing the issue. External pumps are prone to failure due to heat and lack of cooling. How far from the tank is your pump? Leaking/sticking/gummed up injectors could be it too. We had a 91 TPI Vette years ago that was running similar and we found after many hours working on it that the coolant temp sensor was shorted telling the computer the engine was over heating but the gauge showed normal. It ran all sorts of goofy. That car was probably the most finicky TPI car I was ever around.

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40Ford you sure have an interesting selection of cars, one for any occasion - sounds like the way to go. I'll take your suggestion and get my wife out here to check fuel pressure when I'm trying to get it to run. It hasn't run good for a few days now - just coughing & sputtering & backfiring and never taking any throttle.

Mr Natural

I am just thinking out loud, never having trouble-shooted that particular issue: maybe pull the plugs, and swab the cylinders. Wet swab = leaky injector.

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Originally Posted by mr-natural

I have not found out how to troubleshoot which injector is leaking. Do you have any reference to how to trouble shoot that issue?

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My mechanic friend lifted the fuel rail & injectors clear of the intake leaving the injector electrical clips in place, blocked it up so he could see the injector tips and energized the system which puts pump pressure on the rail with the engine off. I think he said that two or three of six were dripping, not at the same rate so I had him replace all six.

Another thought - on my '33 with Edelbrock injection, my Fuel Pressure Regulator failed and was sucking raw fuel through the diaphragm. I called Edelbrock and the Tech knew immediately that my FPR was leaking and sent me a new one. For troubleshooting, I copied this from an OEM forum -
"Did you check the FPR by pulling the vacuum line off while the engine is idling?? Just pulling the vacuum line off may not show if there is leakage past the diaphragm as the fuel will get sucked away by the vacuum and thru the vac line and into the intake. Pull the vac line off while the engine is idling and plug it(the vac hose, not the nipple on the FPR). Let it idle for several minutes and watch the vac nipple on the FPR for any signs of fuel leakage."

Whenever you are checking fuel system problems----------know where a workable fire extingser is nearby--------don't have any vehicle sitting just outside the garage door------------don't have anything between the car your working on and the door-----------just so you can do something if it catches on fire------

we went to help a customer with his first time fire up of a hemi we built----the drive way was blocked outside the door-why not-hemi car had not moved for months------wellllllllllllllllll

^^^^ SAGE advice!!

Well, I've been letting my lumbar fuse for the past two months so now I'm back into it. After lots more farting around I came up with, using the wise advice of a good friend, a distributor problem. A new drive gear and proper shimming to .013" end play I now have and engine that starts and sounds good - for 5 to 10 seconds then it dies. That is with the timing lead disconnected that goes to the ECM. When that lead is reconnected the engine will run longer but still dies after a few minutes. Watching the timing light reveals that the ignition is shutting down to cause the death. I monitored the battery voltage at the distributor and it stays hot during the death. All components in the distributor are new including cap. Fuel pressure holds fine during those few seconds when all is well. Any ideas are welcome. ECM?

Take good care of that back!

Looking back through your first post you said,

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Originally Posted by mr-natural

I believe my grounds are good but I could be missing something there.

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It doesn't cost much to ensure that your grounds are good, and good grounds are critical. I'd say verify your battery to chassis ground, your engine block to chassis ground and your alternator case ground. Star washers are your friend, especially on painted surfaces.
Once your grounds are confirmed if you still have the problem I'm thinking your idea of the ECM being bad may be the culprit.

I'd agree with Mr. Spears, easiest things to check are the grounds, they are crucial to complete the circuit and often overlooked.

You can use a jumper cable right from the battery (-) to a clean metal spot on the motor and see if the motor runs without interruption. Quick and easy way to test.

Well, here's a few more data points. When I said the engine runs after reconnecting the EST bypass that was telling only a part of the story. Here's some more detail. With the EST bypass disconnected I set the timing to about 12° BTDC and the engine would start nicely and run good for 5-10 second and die. While watching the timing mark it is clear that the spark stopped right before the engine quit. So I reconnect the EST bypass and now the engine will stay running if you feather the throttle BUT it is backfiring through the exhaust and the timing is now off scale at about 16° ATDC. All components inside the distributor are new including coil and cap. Disconnecting the EST module does not change a thing. I've gone through all the grounds and they are good. New computer - same same. Should I rip out the damn tuned port and put a carb on?

Well, here's a few more data points. When I said the engine runs after reconnecting the EST bypass that was telling only a part of the story. Here's some more detail. With the EST bypass disconnected I set the timing to about 12° BTDC and the engine would start nicely and run good for 5-10 second and die. While watching the timing mark it is clear that the spark stopped right before the engine quit. So I reconnect the EST bypass and now the engine will stay running if you feather the throttle BUT it is backfiring through the exhaust and the timing is now off scale at about 16° ATDC. All components inside the distributor are new including coil and cap. Disconnecting the EST module does not change a thing. I've gone through all the grounds and they are good. New computer - same same. Should I rip out the damn tuned port and put a carb on?

Mr-Natural, everyone else may recognize "EST" but it's Greek to me. I have absolutely no idea what you mean by "EST Bypass". If you use an acronym it would really help if you'd define it at least once. IMO a good EFI system is an improvement on a carb, but I'd say that for me it makes sense to go with one of the new aftermarket, self learning systems as opposed to trying to band aid an OEM system.

I wish you the best figuring out your system, and truly hope that you get it sorted out.

The latest!!!!!! I gave up and for the first time in the 56 years I've owned this Corvette I brought it into a "guy" who knows L98s. Within the first 5 minutes he determined I had a vacuum leak - turns out my factory runners were warped. He figures it was about a 0.015" gap. He installed a pair of used BBK runners - part of the problem solved. Next was the jumping timing issue - just like I described earlier, one minute stable then all over the place. Believing me that I had totally gone through the distributor he stuck in another used unit - problem solved - well mostly. After the new used distributor the alternator started putting out 16 volts. He stuck in a new rebuilt NAPA alternator as the one that failed was still under warrantee. I picked the car up and while driving home the ammeter was all over the place from discharge to overcharge. Since this "guy" is 38 miles from home and my ride had already left I continued on home. Pulled the alternator and got another rebuild from NAPA. Stuck it in and it too is putting out too much voltage pegging the ammeter - 15.1 volts. Went back and got another one - this one has the same issue that the one I drove home with but not nearly as bad. Ammeter jumping around but not too ridiculous. Ordered a 1 wire regulator for my 10SI alternator and I'll replace that soon. OH, almost forgot, still backfiring through the intake when cold but goes away when warmed up but other than that it runs strong.

Well. I guess that's some progress. Better than zero!
I wonder if the overvoltage / overcurrent is cooking your electronics?

And EVERY time I had an Alternator not regulate itself, there was a failed GROUND involved.