what would cause these rpm fluctuations?
11-30-2009, 01:22 AM
#1
Thread Starter
Join Date: Jul 2009
Location: Florida
Posts: 9
what would cause these rpm fluctuations?
1990 Mustang 5.0 stock engine...stock fuel pump.... pretty much all stock
Manual transmission
When I am driving the vehicle, at highway speeds the rpms will randomly drop typically from 3000 to 2500 and back up again in rapid succession. This is not happening when the vehicle is under acceleration. I'm just cruising along at 80 or 85 mph and the vehicle starts feeling like it is bucking. I drive about 35 miles one way to work and it tended to occur once or twice during the trip. This problem occurred over the course of a few months. Now it has worsened and it is occurring at speeds less then highway and the frequency has increased. It never occurs under 35mph.
Also if I am accelerating hard it has recently started to duplicate the same symptoms. If I am in third gear giving it full acceleration pushing past 4000 rpms the rpms will plummet to 3500 and back up again very rapidly.
The ignition components on the vehicle are newer. Spark plugs, wires, and coil are new. They were changed after the problem developed. The problem has worsened since. I changed the spark plugs and wires as maintenance and someone else recommended the coil as a possible fix so I changed that to no avail.
I am assuming it is fuel related.
the check engine light is not on
any ideas or advice would be appreciated. Thanks!
Manual transmission
When I am driving the vehicle, at highway speeds the rpms will randomly drop typically from 3000 to 2500 and back up again in rapid succession. This is not happening when the vehicle is under acceleration. I'm just cruising along at 80 or 85 mph and the vehicle starts feeling like it is bucking. I drive about 35 miles one way to work and it tended to occur once or twice during the trip. This problem occurred over the course of a few months. Now it has worsened and it is occurring at speeds less then highway and the frequency has increased. It never occurs under 35mph.
Also if I am accelerating hard it has recently started to duplicate the same symptoms. If I am in third gear giving it full acceleration pushing past 4000 rpms the rpms will plummet to 3500 and back up again very rapidly.
The ignition components on the vehicle are newer. Spark plugs, wires, and coil are new. They were changed after the problem developed. The problem has worsened since. I changed the spark plugs and wires as maintenance and someone else recommended the coil as a possible fix so I changed that to no avail.
I am assuming it is fuel related.
the check engine light is not on
any ideas or advice would be appreciated. Thanks!
11-30-2009, 01:54 AM
#2
6th Gear Member
Join Date: Nov 2006
Location: Spfld, MA
Posts: 9,240
first step would be determining whether you're losing fuel or spark, which is sometimes hard to do when the problem is only recreatable under driving conditions.
my first step here would be checking/replacing the fuel filter. after that, if the problem still exists, buy a fuel pressure gauge set, and tape the gauge to the windshield as you drive. when it happens, quickly look at the gauge, and make sure its where it should be.
if it is, its time to move on. at this point, you'd want to check the TFI module and the PIP module, both of which are in direct control of fuel and spark control.
running codes might not be a bad idea, either, as a TPS or MAF issue could cause this as well, but the other ideas are more probable for failure.
my first step here would be checking/replacing the fuel filter. after that, if the problem still exists, buy a fuel pressure gauge set, and tape the gauge to the windshield as you drive. when it happens, quickly look at the gauge, and make sure its where it should be.
if it is, its time to move on. at this point, you'd want to check the TFI module and the PIP module, both of which are in direct control of fuel and spark control.
running codes might not be a bad idea, either, as a TPS or MAF issue could cause this as well, but the other ideas are more probable for failure.
11-30-2009, 01:50 PM
#3
Thread Starter
Join Date: Jul 2009
Location: Florida
Posts: 9
thanks for the info....
gonna change the fuel filter on wednesday and see if that makes any difference...
I wish it replicated the problem at idle on the driveway.... would make it so much easier to figure out.
I'm not looking forward to rigging up a fuel pressure gauge to windshield but I guess my only other choice is the "wait till it finally really breaks and stops running" option that I was so fond of in my youth
gonna change the fuel filter on wednesday and see if that makes any difference...
I wish it replicated the problem at idle on the driveway.... would make it so much easier to figure out.
I'm not looking forward to rigging up a fuel pressure gauge to windshield but I guess my only other choice is the "wait till it finally really breaks and stops running" option that I was so fond of in my youth
11-30-2009, 05:12 PM
#5
Thread Starter
Join Date: Jul 2009
Location: Florida
Posts: 9
alrightie..... hope i solved the problem.... be prepared to be amazed at my ignorance!
so after reading the couple suggestions here and doing some research I decided to go out to the car to make sure everything on the distributor was plugged in nicely. As i pressed the plug in, I noticed the whole distributor shifted. It was quite loose.
I have no idea how to time a car so I just turned the distributor till the car sounded like it was running smooth and snugged the bolt down.
now I've had what I thought was an unrelated problem that someone else told me might have to do with timing. If I drive the car for thirty or fourty minutes.... stop somewhere... like the gas station, where the car is off only for a few minutes and then I try to restart the car it has trouble starting.
so I am hoping against hope that I solved both problems. Likely?
so after reading the couple suggestions here and doing some research I decided to go out to the car to make sure everything on the distributor was plugged in nicely. As i pressed the plug in, I noticed the whole distributor shifted. It was quite loose.
I have no idea how to time a car so I just turned the distributor till the car sounded like it was running smooth and snugged the bolt down.
now I've had what I thought was an unrelated problem that someone else told me might have to do with timing. If I drive the car for thirty or fourty minutes.... stop somewhere... like the gas station, where the car is off only for a few minutes and then I try to restart the car it has trouble starting.
so I am hoping against hope that I solved both problems. Likely?
11-30-2009, 05:33 PM
#6
6th Gear Member
Join Date: Nov 2006
Location: Spfld, MA
Posts: 9,240
possibly. if not, the first thing to diag would be the TFI module, which is bolted to the front of the distributor.
and get it timed correctly. you run the risk of doing serious damage with the timing not right.
and get it timed correctly. you run the risk of doing serious damage with the timing not right.
11-30-2009, 07:20 PM
#9
Thread Starter
Join Date: Jul 2009
Location: Florida
Posts: 9
define trouble starting-----
basically it feels like the battery is very weak.
All of the electrical components in the vehicle work just fine. Only one time it wouldn't start on the first try. I let the car sit for about ten minutes and it refired fine.
basically it feels like the battery is very weak.
All of the electrical components in the vehicle work just fine. Only one time it wouldn't start on the first try. I let the car sit for about ten minutes and it refired fine.
