1988 911 Turbo bucking
03-26-2018, 10:20 AM
#1
Burning Brakes
Thread Starter
1988 911 Turbo bucking
My car had been sitting for 3 or 3 months and yesterday I decided to take it out for a spin. I started the car and let it warm up for 5 minutes. I started driving it and when I was cruising in 2nd gear, it started bucking however it did not stall. I pulled over to let it idle some more and I noticed the the RPMs from jumping from 750 to 1200 and back to 750. I blipped the throttle and it started idling again at 750 so I started driving it again. I had to brake hard at one point and after that it started bucking again. It felt like the car was not getting enough fuel. BTW, all relays are new on the car even the yellow/green relay inside the engine bay. Car has no cat converter and no air box. Could it bad fuel or bad fuel pump?
03-26-2018, 01:59 PM
#3
Burning Brakes
Thread Starter
03-27-2018, 12:01 PM
#4
When you felt the bucking, were you under high boost? If so, then the boost-triggered fuel cutoff switch may have kicked in and shut off the fuel pump, It's a simple sensor that's designed to activate at a designated boost level (around 1.2 bar) to protect against overboost. These switches can fail and shut off the fuel prematurely. If it's of concern, you can bypass the switch to see if things improve. Another distant possibility that the wastegate may not be opening correctly and that it may be "stuck" such that the actual boost is going so high as to trigger the (properly functioning) fuel cutoff switch. But when either of these happens, the bucking is particularly violent, because you go from full boost acceleration to no power at all - alternately.. If the bucking occurs when you're not under significant boost, then it has nothing to do with high boost level or a defective safety switch. And if so, it's also not likely to be from a boost leak. In fact, I can't imagine how a boost leak could cause bucking under any circumstance.
Another possibility is that one of your fuel pumps is dying. This is commonly the front pump, and often happens to cars that sit. Rust and forms and deposits in the bottom of the fuel tank and gets sucked into the front pump, causing it to seize. There is no significant fuel filter prior to that pump (though you can install a small in-line filter yourself). You should take the fuel sender cover off the top of the fuel tank and look in with a flashlight. If you see orange rust in the bottom well of the tank where the fuel pickup is located, than that's a likely culprit. How to get rid of the rust is a whole different issue. But at least you'd know. Another way to diagnose whether it may be the front pump is to remove both fuel pump relays and, with the key on, plug the relays in, one at a time, to see if you can confirm that each pump is running independently. You might find that one of the pumps is not running. But if this is an intermittent problem, this may not be definitive. It might be wise (and cheap) to just replace both relays for good measure, even though they may be relatively new. You could also install a temporary fuel pressure gauge in the system for diagnostic purposes -- and place the gauge in the cockpit so you can observe fuel pressure when the bucking occurs.to see if it has dropped. I don't think I've ever heard of a fuel pump failing intermittently and causing bucking, but I suppose it's possible. Ditto for bad fuel, but perhaps it's possible.
There may be electrical (ignition) causes as well. It would have to be a primary ignition system issue, not secondary. But I can't think of any likely culprits that would cause bucking as you described.
Good luck and let us know what you find.
Another possibility is that one of your fuel pumps is dying. This is commonly the front pump, and often happens to cars that sit. Rust and forms and deposits in the bottom of the fuel tank and gets sucked into the front pump, causing it to seize. There is no significant fuel filter prior to that pump (though you can install a small in-line filter yourself). You should take the fuel sender cover off the top of the fuel tank and look in with a flashlight. If you see orange rust in the bottom well of the tank where the fuel pickup is located, than that's a likely culprit. How to get rid of the rust is a whole different issue. But at least you'd know. Another way to diagnose whether it may be the front pump is to remove both fuel pump relays and, with the key on, plug the relays in, one at a time, to see if you can confirm that each pump is running independently. You might find that one of the pumps is not running. But if this is an intermittent problem, this may not be definitive. It might be wise (and cheap) to just replace both relays for good measure, even though they may be relatively new. You could also install a temporary fuel pressure gauge in the system for diagnostic purposes -- and place the gauge in the cockpit so you can observe fuel pressure when the bucking occurs.to see if it has dropped. I don't think I've ever heard of a fuel pump failing intermittently and causing bucking, but I suppose it's possible. Ditto for bad fuel, but perhaps it's possible.
There may be electrical (ignition) causes as well. It would have to be a primary ignition system issue, not secondary. But I can't think of any likely culprits that would cause bucking as you described.
Good luck and let us know what you find.
03-28-2018, 02:09 PM
#5
Burning Brakes
Thread Starter
When you felt the bucking, were you under high boost? If so, then the boost-triggered fuel cutoff switch may have kicked in and shut off the fuel pump, It's a simple sensor that's designed to activate at a designated boost level (around 1.2 bar) to protect against overboost. These switches can fail and shut off the fuel prematurely. If it's of concern, you can bypass the switch to see if things improve. Another distant possibility that the wastegate may not be opening correctly and that it may be "stuck" such that the actual boost is going so high as to trigger the (properly functioning) fuel cutoff switch. But when either of these happens, the bucking is particularly violent, because you go from full boost acceleration to no power at all - alternately.. If the bucking occurs when you're not under significant boost, then it has nothing to do with high boost level or a defective safety switch. And if so, it's also not likely to be from a boost leak. In fact, I can't imagine how a boost leak could cause bucking under any circumstance.
Another possibility is that one of your fuel pumps is dying. This is commonly the front pump, and often happens to cars that sit. Rust and forms and deposits in the bottom of the fuel tank and gets sucked into the front pump, causing it to seize. There is no significant fuel filter prior to that pump (though you can install a small in-line filter yourself). You should take the fuel sender cover off the top of the fuel tank and look in with a flashlight. If you see orange rust in the bottom well of the tank where the fuel pickup is located, than that's a likely culprit. How to get rid of the rust is a whole different issue. But at least you'd know. Another way to diagnose whether it may be the front pump is to remove both fuel pump relays and, with the key on, plug the relays in, one at a time, to see if you can confirm that each pump is running independently. You might find that one of the pumps is not running. But if this is an intermittent problem, this may not be definitive. It might be wise (and cheap) to just replace both relays for good measure, even though they may be relatively new. You could also install a temporary fuel pressure gauge in the system for diagnostic purposes -- and place the gauge in the cockpit so you can observe fuel pressure when the bucking occurs.to see if it has dropped. I don't think I've ever heard of a fuel pump failing intermittently and causing bucking, but I suppose it's possible. Ditto for bad fuel, but perhaps it's possible.
There may be electrical (ignition) causes as well. It would have to be a primary ignition system issue, not secondary. But I can't think of any likely culprits that would cause bucking as you described.
Good luck and let us know what you find.
Another possibility is that one of your fuel pumps is dying. This is commonly the front pump, and often happens to cars that sit. Rust and forms and deposits in the bottom of the fuel tank and gets sucked into the front pump, causing it to seize. There is no significant fuel filter prior to that pump (though you can install a small in-line filter yourself). You should take the fuel sender cover off the top of the fuel tank and look in with a flashlight. If you see orange rust in the bottom well of the tank where the fuel pickup is located, than that's a likely culprit. How to get rid of the rust is a whole different issue. But at least you'd know. Another way to diagnose whether it may be the front pump is to remove both fuel pump relays and, with the key on, plug the relays in, one at a time, to see if you can confirm that each pump is running independently. You might find that one of the pumps is not running. But if this is an intermittent problem, this may not be definitive. It might be wise (and cheap) to just replace both relays for good measure, even though they may be relatively new. You could also install a temporary fuel pressure gauge in the system for diagnostic purposes -- and place the gauge in the cockpit so you can observe fuel pressure when the bucking occurs.to see if it has dropped. I don't think I've ever heard of a fuel pump failing intermittently and causing bucking, but I suppose it's possible. Ditto for bad fuel, but perhaps it's possible.
There may be electrical (ignition) causes as well. It would have to be a primary ignition system issue, not secondary. But I can't think of any likely culprits that would cause bucking as you described.
Good luck and let us know what you find.
03-28-2018, 11:23 PM
#6
Burning Brakes
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One of the fuel pumps not pumping...due to an intermittently bad relay perhaps...might cause some bucking when you apply throttle. Next time it happens when all warmed up, pop the hood and pull one of the pump relays at a time and see if the rpms change. If the rpms don't change, then you've found it. The term "bucking" can be interpreted many ways. Tough diagnosis without being there.
03-31-2018, 10:21 AM
#7
uninformed gas bag
(contemplating on whether gas bag is one or two words)
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(contemplating on whether gas bag is one or two words)
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Could be so many things. You already stated that it has new relays. Make sure the contacts are clean and snug.
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04-01-2018, 02:51 PM
#8
Burning Brakes
Thread Starter
Not taking any chances. I am having the car towed to my mechanic for repairs. Will keep the board posted of the results.
04-01-2018, 05:13 PM
#9
uninformed gas bag
(contemplating on whether gas bag is one or two words)
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(contemplating on whether gas bag is one or two words)
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If I may ask who do you use for service. Not happy as of late with the shop in central NJ I've used for many years.
04-01-2018, 08:54 PM
#10
Burning Brakes
Thread Starter
Carrera Car Shop (Peter Potzinger). Huntington Station, NY
04-02-2018, 07:01 AM
#11
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Fuel pump
hi
Classic failing fuel pump problem. I concur with many of the other guys.This turbo has two fuel pumps. It could be either, usually back one.
The symptoms usually occur under load or acceleration.
Both pumps will be old so swop both for future reliability. Expensive on parts but low on repair shop hours. Can be done in 1 hour.
Paul
ex 1989 Turbo UK limited edition owner
PS Please let us know how you got on, so many don’t.
Classic failing fuel pump problem. I concur with many of the other guys.This turbo has two fuel pumps. It could be either, usually back one.
The symptoms usually occur under load or acceleration.
Both pumps will be old so swop both for future reliability. Expensive on parts but low on repair shop hours. Can be done in 1 hour.
Paul
ex 1989 Turbo UK limited edition owner
PS Please let us know how you got on, so many don’t.
04-02-2018, 08:08 AM
#12
Burning Brakes
Thread Starter
hi
Classic failing fuel pump problem. I concur with many of the other guys.This turbo has two fuel pumps. It could be either, usually back one.
The symptoms usually occur under load or acceleration.
Both pumps will be old so swop both for future reliability. Expensive on parts but low on repair shop hours. Can be done in 1 hour.
Paul
ex 1989 Turbo UK limited edition owner
PS Please let us know how you got on, so many don’t.
Classic failing fuel pump problem. I concur with many of the other guys.This turbo has two fuel pumps. It could be either, usually back one.
The symptoms usually occur under load or acceleration.
Both pumps will be old so swop both for future reliability. Expensive on parts but low on repair shop hours. Can be done in 1 hour.
Paul
ex 1989 Turbo UK limited edition owner
PS Please let us know how you got on, so many don’t.
04-09-2018, 08:04 PM
#13
Burning Brakes
Thread Starter
I just got word that the problem was a bad alternator which is being replaced.
04-09-2018, 11:05 PM
#14
Burning Brakes
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Hope that turns out to be the issue. Keep us informed for the general knowledge bank.
04-09-2018, 11:21 PM
#15
Burning Brakes
Thread Starter
Well I wouldn't have thought that. One would think you would have seen other signs as well, such as your tach jumping all over the place. Unless the alternator was so bad it was shorting out and grounding power to the coil or CDI sporadically...but then, you would get a backfire. Hmmm, these cars are brain teasers.
Hope that turns out to be the issue. Keep us informed for the general knowledge bank.
Hope that turns out to be the issue. Keep us informed for the general knowledge bank.
Mine is surely a brain teaser and trouble maker. Has left me stranded a few too many times but she is beautiful and a delight to drive not mention how everyone goes crazy when they see her on the road.
