need help diagnosing a intermittent starting problem
Thread Starter
Burning Brakes
Joined: Oct 2007
Posts: 1,093
Likes: 33
From: Morristown TN
need help diagnosing a intermittent starting problem
The car is my wife's 86 stock L98 automatic with the cast iron heads.
We have had it for 3 years and it is bullet proof considering how she always seemed to have problems on every other car she has had new or used. We bought her a new Neon that spent more time at the dealer than it did in our drive.
She has had only basic problems with the Vette, the worst being a fuel pump with some bad wiring 4 months ago. Pump and filter were replaced at that time.
She recently complained it was getting hard to start at times but when I tried it started right up every time.
I had already had a new exhaust scheduled to be installed while we were on vacation. I took it to a trusted mechanic and had a full tune up anyway.
Spark plug's, wire's, distributor cap, rotor button, pcv valve, oil change, air filter, plus check for other things. He found some dry rotted vacuum lines but that was all. He could not repeat her problem.
I trust this guy fully, he works on my C5 when it is something I can not or do not have time to do plus he is fully certified and not a shade-tree mechanic.
Fast forward today, I drove it all day and it did it to me.
It would crank over for 5 seconds and not fire then the next time I cranked it over it started right away.
It would not do it every time.
I know it is not the security system because that was bypassed when a remote start was installed 2 years ago. It does the same thing with the remote start with the exception that it cranks for about 7 seconds and then starts but will sometimes die but will start right up next time.
It does it at different times, it normally will start 1st thing in the morning but after sitting 8 hours it will not according to her.
any ideas?
We have had it for 3 years and it is bullet proof considering how she always seemed to have problems on every other car she has had new or used. We bought her a new Neon that spent more time at the dealer than it did in our drive.
She has had only basic problems with the Vette, the worst being a fuel pump with some bad wiring 4 months ago. Pump and filter were replaced at that time.
She recently complained it was getting hard to start at times but when I tried it started right up every time.
I had already had a new exhaust scheduled to be installed while we were on vacation. I took it to a trusted mechanic and had a full tune up anyway.
Spark plug's, wire's, distributor cap, rotor button, pcv valve, oil change, air filter, plus check for other things. He found some dry rotted vacuum lines but that was all. He could not repeat her problem.
I trust this guy fully, he works on my C5 when it is something I can not or do not have time to do plus he is fully certified and not a shade-tree mechanic.
Fast forward today, I drove it all day and it did it to me.
It would crank over for 5 seconds and not fire then the next time I cranked it over it started right away.
It would not do it every time.
I know it is not the security system because that was bypassed when a remote start was installed 2 years ago. It does the same thing with the remote start with the exception that it cranks for about 7 seconds and then starts but will sometimes die but will start right up next time.
It does it at different times, it normally will start 1st thing in the morning but after sitting 8 hours it will not according to her.
any ideas?
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
I had an early 86 like yours.
Fuel pump: Becomes energized in one of two ways. Either the fuel pump relay energizes the pump for 2-3 seconds when the key is turned to ON and will stay energized as long as the ECM sees ignition pulses from the distributor (engine cranking) OR
Should the relay fail, there's a built in back up with an oil pressure switch (near firewall screwed into block) which will energize the fuel pump once it receives around 3 or 4 psi of oil pressure.
Seems that your engine is energizing the pump via oil pressure which would account for the longish start times.
Way to check is to listen. Turn the key to ON while listening for the hum of the fuel pump in the gas tank. This hum will only last 2-3 seconds you've gotta be quick. No hum, I'd check the FP relay.
You can run 12V to the relay to check it. If 12v makes the pump energize (hum), I'd definitely zero in on a bad relay. IIRC a couple of relays under the hood are the same and one of them can be swapped with the FP relay to check if that one will turn on the pump. I've forgotten which one, so maybe we'd better just let that option go for now.
Another test is fuel priming. Car sitting overnight loses fuel priming which is what the 2-3 second pump activation is suppose to take car of. You can try turning the key to ON, wait the 2-3 seconds for the pump to turn off, then turn the key OFF and wait a several seconds before trying to start the engine. On the start attempt the system should be primed from the first turn ON. If it starts immediately I'd be thinking priming as the cause.
Let us know how your tests turn out.
Jake
Fuel pump: Becomes energized in one of two ways. Either the fuel pump relay energizes the pump for 2-3 seconds when the key is turned to ON and will stay energized as long as the ECM sees ignition pulses from the distributor (engine cranking) OR
Should the relay fail, there's a built in back up with an oil pressure switch (near firewall screwed into block) which will energize the fuel pump once it receives around 3 or 4 psi of oil pressure.
Seems that your engine is energizing the pump via oil pressure which would account for the longish start times.
Way to check is to listen. Turn the key to ON while listening for the hum of the fuel pump in the gas tank. This hum will only last 2-3 seconds you've gotta be quick. No hum, I'd check the FP relay.
You can run 12V to the relay to check it. If 12v makes the pump energize (hum), I'd definitely zero in on a bad relay. IIRC a couple of relays under the hood are the same and one of them can be swapped with the FP relay to check if that one will turn on the pump. I've forgotten which one, so maybe we'd better just let that option go for now.
Another test is fuel priming. Car sitting overnight loses fuel priming which is what the 2-3 second pump activation is suppose to take car of. You can try turning the key to ON, wait the 2-3 seconds for the pump to turn off, then turn the key OFF and wait a several seconds before trying to start the engine. On the start attempt the system should be primed from the first turn ON. If it starts immediately I'd be thinking priming as the cause.
Let us know how your tests turn out.
Jake
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
When it won't start, THAT is the time to do some diagnostics to find out what is missing. Auto engines require hot spark and spark at the right time, fuel and air and in the correct ratio and cylinder compression in order to start and continue running. Use a timing light to see if timing is ok, bring a grounded wire close to an unplugged spark plug boot to see if you have a snappy at least 1/2" spark while cranking. Measure fuel pressure, it should be 35-42 psi and it should hold up with the ign off. Use a noid light on each bank of injectors to see if the injectors are being pulsed. See if it will start with starting fluid, this tells you if you have a fuel delivery problem. Cylinder pressure is rarely intermittent, but it won't hurt to measure it.
