Hard Engine vibration 1988 928s4
12-23-2003, 12:34 AM
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Hard Engine vibration 1988 928s4
2 months ago I purchased a 1988 S4, I had test driven this car and it was excellent at all power levels. When I drove it home (120 miles) it ran well until I slowed down close to the house. When I applied light power I got a loud vibration. The vibration was violent and seemed to be causing the exhaust pipe to hit the frame. I noticed that the at low speed and at idle the engine ran rough, although is reved OK. I suspected that the cam had jumped timing, In short I was scared real engine damage had been done. Long story short, it was not cam timing. The cams are on the mark and I have 155 psi compression on every cylinder. When this car is driven this vibration is erratic sometimes the engine performs flawlessly with no vibration. Then ten minutes later the problem is back. It will occur under light acceleration starting from a dead start, or when you barely accelerate at 70 MPH. At other times there is no problem at all. When it occurs the vibration is harsh and loud. Full throttle for a couple of seconds clears the problem. The oil is clean so the crank hasen't spun a bearing. Under full power the engine performs well and the transmission shifts fine. So what gives?? This condition feels like some of the cylinders are firing out of time or predetonating or just not getting fuel. 928 INTL says it may be a failing air flow sensor, but at $500.00 to replace, I am just trying to get some experienced opinions. this problem might go away for 30 or 40 miles but then returns. Anybody got any ideas.
12-23-2003, 12:43 AM
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Have you checked all the connections on ignition and sensor wires?
Do you see sparks from the ignition wires at night?
Are the distributor caps and rotors original Bosch parts?
Just a few random suggestions...
Do you see sparks from the ignition wires at night?
Are the distributor caps and rotors original Bosch parts?
Just a few random suggestions...
12-23-2003, 04:36 AM
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You're discription sounds somewhat similar to a problem I just fixed on my 87 S4 with the exception that I never noticed it at a specific speed other than slow...
The short answer: Check your passenger side radiator fan, and look carefully at the hoses around it, particularly the air intake hose that originates in the center of the radiator.
The following can be ignore, it's my long-*** original answer that I didn't want to waste after typing that much and thought someone might enjoy reading
I'd be driving along and all of a sudden I'd get what felt like a cavatation from the engine. Typically I'd notice it at slower speeds or when I was stopped at a light. It could just go away and the car would be purring great, then BAM! My engine felt like it was going to fall out. It was very disconcerting and like you I imediately had visions of pistons hitting valves. A better visualization would be to imagin someone magically making all your engine mounts disapear.
What was the problem? There is an air intake hose that's stuck to the back-top-center of the radiator and follows the radiator fan shroud from the top down to the passenger side. The shroud doesn't cover the fans entirely, there is about 1/2" of fan that sticks out the back of the shroud. The air intake tube wraps around it but there is nothing holding it in place. The tube vibrated or moved from beside the shroud to just behind the edge of the shroud and was barely touching the rear of the passenger side radiator fan.
The tube didn't interfere with the fans operation enough to make it stop, but it did put enough pressure on the fan to make it "wobble" and from friction, start wearing a hole through the air tube.
When I slowed down, the engine was hot and the fans kicked in. The car felt like it was shaking apart... Then the feeling would just go away. The noise and the vibration kept me from even being able to hear the fans.
I eventually found it when a good song was playing on the radio so I didn't turn the ignition all the way off. I knew the engine was off, yet it sounded exactly like it was still running! Sure enough, it was the fans kicking in because there was no air coming in from the forward momentum of the car.
The short answer: Check your passenger side radiator fan, and look carefully at the hoses around it, particularly the air intake hose that originates in the center of the radiator.
The following can be ignore, it's my long-*** original answer that I didn't want to waste after typing that much and thought someone might enjoy reading
I'd be driving along and all of a sudden I'd get what felt like a cavatation from the engine. Typically I'd notice it at slower speeds or when I was stopped at a light. It could just go away and the car would be purring great, then BAM! My engine felt like it was going to fall out. It was very disconcerting and like you I imediately had visions of pistons hitting valves. A better visualization would be to imagin someone magically making all your engine mounts disapear.
What was the problem? There is an air intake hose that's stuck to the back-top-center of the radiator and follows the radiator fan shroud from the top down to the passenger side. The shroud doesn't cover the fans entirely, there is about 1/2" of fan that sticks out the back of the shroud. The air intake tube wraps around it but there is nothing holding it in place. The tube vibrated or moved from beside the shroud to just behind the edge of the shroud and was barely touching the rear of the passenger side radiator fan.
The tube didn't interfere with the fans operation enough to make it stop, but it did put enough pressure on the fan to make it "wobble" and from friction, start wearing a hole through the air tube.
When I slowed down, the engine was hot and the fans kicked in. The car felt like it was shaking apart... Then the feeling would just go away. The noise and the vibration kept me from even being able to hear the fans.
I eventually found it when a good song was playing on the radio so I didn't turn the ignition all the way off. I knew the engine was off, yet it sounded exactly like it was still running! Sure enough, it was the fans kicking in because there was no air coming in from the forward momentum of the car.
12-23-2003, 11:30 PM
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Check the coil wires carefully, and move them away form any metal or wire harnesses. Check the tops of the coils, the distributo caps and the rotors.
12-23-2003, 11:46 PM
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Wally
Thanks for the input. I will have time Wednesday to take a closer look at the rotors coils and wires. The motor looks like it is almost new, very clean, no oxidation, zinc plated parts still shiny and new looking.
When I have this problem it almost feels like the plugs are firing out of time. It is much more than a cylider just missing. I mean it is almost like it is hammering. The plugs by the way are perfect. I will also look at the engine running in the dark to check for arcing. Thanks Again
PS just moving my mom and brother up from Cumming Ga.
Thanks for the input. I will have time Wednesday to take a closer look at the rotors coils and wires. The motor looks like it is almost new, very clean, no oxidation, zinc plated parts still shiny and new looking.
When I have this problem it almost feels like the plugs are firing out of time. It is much more than a cylider just missing. I mean it is almost like it is hammering. The plugs by the way are perfect. I will also look at the engine running in the dark to check for arcing. Thanks Again
PS just moving my mom and brother up from Cumming Ga.
