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Help! 1986 Vette Dies 20-30 minutes after starting

Doctorbatman

Member
Joined
Apr 28, 2019
Messages
5
Location
USA
Corvette
1986
Hi All,

1986 Base Corvette, start it, let it idle or drive it, around 20-30 minutes it shuts down. Attempted restart and at time idles another 5-10 minutes then shuts itself down or won't restart. When attempt restart will only start if slightly open throttle, won't idle but can force a high idle around 1200 RPM. Won't rev higher, just bogs and dies unless let up of throttle slightly. Backfires through the intake like it's lean.

Let it sit a few hours, will repeat same action. Acted this way since I got the car 1.5 years ago. So far I've only driven it 20 minutes at a time. When I test drove the car before purchase I did not drive it 20 minutes so was not aware of the issue when I did buy it.

Replaced fuel pump, fuel filter, checked supply and return fuel lines are clear, replaced fuel pressure regulator with adjustable cover and set pressure to 42 psi at idle, removed and cleaned fuel rail, replaced all injectors, tested all injectors are flashing with test light before and after car dies after 20 minutes.

Replaced Mas Airflow sensor twice, replaced MAS Power and Burn off relays, replaced bad rear cooling fan motor, replaced fan relay, replaced, Idle Air Control Valve, all engine temp sensors, throttle position sensor (set. to .52 volts at idle), replaced entire distributor assembly, replaced ECM. Replaced with quality spark plug wires and AC Delco spark plugs.

Tested voltage drop for all ground wires from ECM - all in spec. Removed, cleaned and re-secured all body ground straps with dielectric grease.

Engine timing set to 6 degrees BTDC when disconnected to computer, when computer is reconnected, timing averages 24 degrees BTDC and goes to about 32 degrees BTDC when revving engine. Tested and dielectric greased all fuses in panel.

Tried powering distributor and fuel pump directly from battery bypassing wire harness and engine still dies at 20 minutes.

I can only suspect a problem somewhere in the car's wiring harness since I've replaced all 4 temperature sensors, TPS, Mass Airflow Sensor and O2 sensor as well as new ECM and I borrowed another ECM with PROM from another 1986 Corvette and tried running it. (As well as all the other stuff I've mentioned.)

I connected a second battery in case of too much battery drain when the cooling fan kicks on but no help.

Since the ECM takes in sensory data, makes the decision on how much fuel to use and when to spark, and I've replaced every sensor and connecting pigtail, I can't imaging where the confusion is coming from except from the wiring harness somewhere in the car.

Any other thoughts?


At this point I've had 7 mechanics (only lousy ones since a good mechanic might take all this fun out of it - just kidding of course) evaluate the car with the only remaining suggestion to remove fuel injection system and distributor and replace with carb and standard HEI vacuum advance distributor. In the absence of any new diagnostic info, that's what I'll be doing next week.

So far I've not found any solutions from searching on-line.

Any last ditch recommendations would be greatly appreciated,

Thank you all,

Doc
 
Joined
Jan 31, 2005
Messages
412
Location
Somewhere in Red Wings land
Corvette
1989 Bright Red Convertible (gone) 98 Jaguar XK8
Hi All,

1986 Base Corvette, start it, let it idle or drive it, around 20-30 minutes it shuts down. Attempted restart and at time idles another 5-10 minutes then shuts itself down or won't restart. When attempt restart will only start if slightly open throttle, won't idle but can force a high idle around 1200 RPM. Won't rev higher, just bogs and dies unless let up of throttle slightly. Backfires through the intake like it's lean.

Any last ditch recommendations would be greatly appreciated,

Thank you all,

Doc

Just a random odd thought,,,,, is there something floating around in the tank restricting or blocking the inlet on the fuel pump after it runs for a while?
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Hi All,

1986 Base Corvette, start it, let it idle or drive it, around 20-30 minutes it shuts down. Attempted restart and at time idles another 5-10 minutes then shuts itself down or won't restart. When attempt restart will only start if slightly open throttle, won't idle but can force a high idle around 1200 RPM. Won't rev higher, just bogs and dies unless let up of throttle slightly. Backfires through the intake like it's lean.

Let it sit a few hours, will repeat same action. Acted this way since I got the car 1.5 years ago. So far I've only driven it 20 minutes at a time. When I test drove the car before purchase I did not drive it 20 minutes so was not aware of the issue when I did buy it.

Replaced fuel pump, fuel filter, checked supply and return fuel lines are clear, replaced fuel pressure regulator with adjustable cover and set pressure to 42 psi at idle, removed and cleaned fuel rail, replaced all injectors, tested all injectors are flashing with test light before and after car dies after 20 minutes.

Replaced Mas Airflow sensor twice, replaced MAS Power and Burn off relays, replaced bad rear cooling fan motor, replaced fan relay, replaced, Idle Air Control Valve, all engine temp sensors, throttle position sensor (set. to .52 volts at idle), replaced entire distributor assembly, replaced ECM. Replaced with quality spark plug wires and AC Delco spark plugs.

Tested voltage drop for all ground wires from ECM - all in spec. Removed, cleaned and re-secured all body ground straps with dielectric grease.

Engine timing set to 6 degrees BTDC when disconnected to computer, when computer is reconnected, timing averages 24 degrees BTDC and goes to about 32 degrees BTDC when revving engine. Tested and dielectric greased all fuses in panel.

Tried powering distributor and fuel pump directly from battery bypassing wire harness and engine still dies at 20 minutes.

I can only suspect a problem somewhere in the car's wiring harness since I've replaced all 4 temperature sensors, TPS, Mass Airflow Sensor and O2 sensor as well as new ECM and I borrowed another ECM with PROM from another 1986 Corvette and tried running it. (As well as all the other stuff I've mentioned.)

I connected a second battery in case of too much battery drain when the cooling fan kicks on but no help.

Since the ECM takes in sensory data, makes the decision on how much fuel to use and when to spark, and I've replaced every sensor and connecting pigtail, I can't imaging where the confusion is coming from except from the wiring harness somewhere in the car.

Any other thoughts?


At this point I've had 7 mechanics (only lousy ones since a good mechanic might take all this fun out of it - just kidding of course) evaluate the car with the only remaining suggestion to remove fuel injection system and distributor and replace with carb and standard HEI vacuum advance distributor. In the absence of any new diagnostic info, that's what I'll be doing next week.

So far I've not found any solutions from searching on-line.

Any last ditch recommendations would be greatly appreciated,

Thank you all,

Doc



It looks like you have done everything but diagnosis. When the vehicle is not running you need to find what is missing, fuel, spark, timing, or compression, and not just throw parts at it which will just make things worse. Find out what parameters are missing with the problem active and diagnose from there.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
When it dies, do you know what causes it? That is, does the engine quit because of fuel starvation or does it quit because of an electrical issue?

What is the fuel pressure when you test it according to the Factory Service Manual?

Have you changed the ignition module inside the distributor?
 
Last edited:

Doctorbatman

Member
Joined
Apr 28, 2019
Messages
5
Location
USA
Corvette
1986
Thank you all for your suggestions.
I'll try to go through all the points of the posts above with my results so far.
I have a Snap On OBD1 MT2500 scanner that has essentially been connected to the car for the last 2 years.
2 years ago I purchased GM original Corvette shop manuals and have been referencing them throughout the process.



The computer throws no codes.


I've Ohm tested the original and 2 replacement Coolant Temp Sensors - all are within spec.



The original fan motor burned out a year ago and I replaced it with a Delco motor.
I also replaced the fan relays.
I removed, cleaned and grease the fan and all other under hood and body grounds.
I conducted a voltage drop test from the battery to ground on engine and chassis.
I conducted voltage drop tests from the ECM harness.
Every fuse in passenger side fuse panel and auxiliary fuse panel was removed, sockets and fuses cleaned, dielectric greased, reinstalled and tested.
Installed new distributor 2 weeks ago with all new pre-installed components.
Tried running engine with fuel cap on and off. Visually and audibly confirmed ample return fuel flow into tank.
Retained a Snap-on fuel pressure gauge on fuel rail to confirm 42 PSI constant fuel pressure before engine start, while engine is running and after engine (appears to suddenly go lean) and then dies.
I've confirmed computer read going from open to closed loop as it shows on the Snap-on scanner.
Scanner shows O2 sensor reads alternating "rich/lean".
I replaced the Intake Air Control Valve and confirmed its function on the scanner.
Have replaced the O2 sensor - twice. The second being a heated unit to assure it gets up to full temp.
In addition to testing the battery, I've run the engine with a second battery connected as well as a battery booster to assure stable voltage when cooling fan cycles on/off. Engine still quits.
9th injector - I've run it with the connector removed. No change, still dies. Seems lean, not rich when it does.
Coil - I changed it twice before changing out the entire distributor, cap, rotor coil, etc. No improvement.
Main fusable link terminal - I removed all wires, terminal post and terminals, cleaned and reinstalled.
Electrical connectors in harness for possible corrosion - I removed all sensor harness pigtails/connectors, and replaced them with new with solder/heat shrink repair. Only the original fuel injector connectors remain, which were cleaned and lubed with di-electric grease when I replaced the injectors.
Today, I bypassed the factory wiring and went directly from the Engine Coolant Temp sensor to the ECM inputs to eliminate any chassis wiring damage - no improvement.


Any other suggestions short of using the car for target practice is appreciated.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Again, save yourself a lot of time and money and find out what you do not have when the engine will not start, and then start your diagnosis. Fuel pressure is one item, but fuel FLOW is more important. If it turns out to be a fuel issue make sure you check the fuel flow which should be about 1qt of fuel in 30 seconds.
 

Doctorbatman

Member
Joined
Apr 28, 2019
Messages
5
Location
USA
Corvette
1986
She moves a LOT of fuel, I can shine a light in the tank and watch a plentiful volume of fuel flowing back into the tank via the return. All the while maintaining 42 PSI at the fuel rail whether the engine is running or not.

The main issue is neither myself or anyone else CAN figure out what it is not doing when it won't run.

Just today had another long time professional mechanic assess the car and was unable to come up with anything other than to check the valve adjustment. His suggestion if that is OK was "put an Edelbrock carb and intake on it, tuck the wire harness aside until someone has another idea."

Thank you for your continued suggestions.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
She moves a LOT of fuel, I can shine a light in the tank and watch a plentiful volume of fuel flowing back into the tank via the return. All the while maintaining 42 PSI at the fuel rail whether the engine is running or not.

The main issue is neither myself or anyone else CAN figure out what it is not doing when it won't run.

Just today had another long time professional mechanic assess the car and was unable to come up with anything other than to check the valve adjustment. His suggestion if that is OK was "put an Edelbrock carb and intake on it, tuck the wire harness aside until someone has another idea."

Thank you for your continued suggestions.[/QUOT




Suggesting that you install a carb is not a professional suggestion, it's an insult.

Find a real professional tech and figure out what is missing if you are not able to. As long as the problem is active, finding the missing event should not be that difficult for someone who knows what they are doing. Now finding why that missing event is happening could be, especially after installing all those aftermarket parts.
 

navy2kcoupe

Well-known member
Joined
Dec 16, 2006
Messages
799
Location
West Central FL and SE Mass.
Corvette
2000 Navy Blue Coupe A4 Z51
Have you given any thought to a plugged cat or cats?
Andy
 
Joined
Jan 31, 2005
Messages
412
Location
Somewhere in Red Wings land
Corvette
1989 Bright Red Convertible (gone) 98 Jaguar XK8
Have you given any thought to a plugged cat or cats?
Andy

Come to think about it, my 89 had very similar symptoms, I had forgotten about that. Turns out the pre cats were plugged. The mechanics at a Corvette specialty shop in Colorado Springs knew exactly what the issue was when I described the symptoms. I replaced the y pipe with an "off road" y pipe and all the issues were solved.
 
Joined
Jan 31, 2005
Messages
412
Location
Somewhere in Red Wings land
Corvette
1989 Bright Red Convertible (gone) 98 Jaguar XK8
Any new developments? Did you check the cats and precats?
 

Doctorbatman

Member
Joined
Apr 28, 2019
Messages
5
Location
USA
Corvette
1986
Thanks, everyone, for all your suggestions.
I looked out my office window into my garage last Friday to see flames poring out of the interior.
I got the fire out with 5 extinguishers.
Everything inside is burned and melted so I obviously won't be working on the original problem anymore.


Doc
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Damn dude, sorry to hear that.

How is your house/garage?
 

Doctorbatman

Member
Joined
Apr 28, 2019
Messages
5
Location
USA
Corvette
1986
I was sitting in my office in my house and happened to look out across my driveway into my open garage where the Vette was parked and saw orange flames coming out of the interior. The top was down and the hood open. Flames were on both sides of the firewall so I don't know where they started. Unbelievable! 5 fire extinguishers and a garden hose later and I got the fire out but the wiring and interior is gone.I'll keep you posted. Still awaiting all the insurance stuff.
 

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