Strange low-RPM Stutter...
06-19-2002 | 06:42 PM
#1
Thread Starter
Racer
Joined: May 2002
Posts: 251
Likes: 0
From: Tampa, Florida
Strange low-RPM Stutter...
I am experiencing an odd performance issue.
My 95 C4 with 54k miles is occasionally stuttering, stammering and losing power.
The car idles at about 900 cold and drops to about 650-700 when warm. It idles smooth except for a 50 RPM bump accompanied by a shutter about every 5 seconds or so.
If I accelerate smoothly through 1st, 2nd and into 3rd -- shifting at 3,500 RPMs -- it is smooth and solid as a rock. If I back off on the throttle to around 2,800 RPMs or lower (pulling up behind slower traffic) after about 10 seconds the engine starts stuttering almost like weak compression braking and drops about 500 RPMs while choking the whole time. After the 10-second episode it seems to “catch” itself and regulate back to the appropriate RPMs for the gear and throttle position. Or, if I depress the clutch and blip the throttle it seems to “clear” itself and go back to normal running.
I changed distributor caps, rotors, plugs, and plug wires (fun job.) No impact. Also cleaned ISV and air filter.
Any thoughts on what to try next?
Thanks,
My 95 C4 with 54k miles is occasionally stuttering, stammering and losing power.
The car idles at about 900 cold and drops to about 650-700 when warm. It idles smooth except for a 50 RPM bump accompanied by a shutter about every 5 seconds or so.
If I accelerate smoothly through 1st, 2nd and into 3rd -- shifting at 3,500 RPMs -- it is smooth and solid as a rock. If I back off on the throttle to around 2,800 RPMs or lower (pulling up behind slower traffic) after about 10 seconds the engine starts stuttering almost like weak compression braking and drops about 500 RPMs while choking the whole time. After the 10-second episode it seems to “catch” itself and regulate back to the appropriate RPMs for the gear and throttle position. Or, if I depress the clutch and blip the throttle it seems to “clear” itself and go back to normal running.
I changed distributor caps, rotors, plugs, and plug wires (fun job.) No impact. Also cleaned ISV and air filter.
Any thoughts on what to try next?
Thanks,
06-19-2002 | 07:35 PM
#2
Burning Brakes
Joined: May 2001
Posts: 967
Likes: 1
One guess would be the fuel filter, but I would think you’d need to be at higher rev’s for a partially blocked fuel filter to be a problem.
Another guess is the low speed surging problem that is common with these cars, but it seems you’re at higher revs than that problem. Here is something I posted on this problem a long time ago:
[quote]Originally posted by tom_993:
<strong>
As for the surging, I understand this is caused by the ECU cutting fuel and adding it back in. What happens is that the ECU cuts fuel anytime the throttle is closed above about 1500-1800 RPM. This is for both mileage and emissions. When this happens, you get compression braking from the engine and you lurch forward. Anything that causes the fuel to come back on (touch the throttle or rev's drop below the limit), will cause strong acceleration and you lurch backwards. Sometimes you get in this on-off-on cycle and the only way to break it is to push the clutch in. This does not happen when you accelerate smoothly, through first and into second. It only happens when you need to modulate the throttle.
</strong><hr></blockquote>
Another guess is the low speed surging problem that is common with these cars, but it seems you’re at higher revs than that problem. Here is something I posted on this problem a long time ago:
[quote]Originally posted by tom_993:
<strong>
As for the surging, I understand this is caused by the ECU cutting fuel and adding it back in. What happens is that the ECU cuts fuel anytime the throttle is closed above about 1500-1800 RPM. This is for both mileage and emissions. When this happens, you get compression braking from the engine and you lurch forward. Anything that causes the fuel to come back on (touch the throttle or rev's drop below the limit), will cause strong acceleration and you lurch backwards. Sometimes you get in this on-off-on cycle and the only way to break it is to push the clutch in. This does not happen when you accelerate smoothly, through first and into second. It only happens when you need to modulate the throttle.
</strong><hr></blockquote>
06-19-2002 | 07:47 PM
#3
RL Technical Advisor
Joined: May 2001
Posts: 11,871
Likes: 66
From: Portland Oregon
Hi Jack:
Sorry to hear of your troubles.
As you have already replaced all the normal stuff and cleaned the ISV and its connector, I'd venture that this might be electrical in nature. By all means, I'd change the fuel filter just in case that you happened to have received a contaminated load of fuel that has partially plugged the thing up.
My best guess is two things:
1) Faulty/ intermittent DME relay. This affects the as well as fuel pressure,...
2) Battery/Alternator issue. Any variations in the operating voltage of the car will make for symptoms like these (and worse). I'd check that harness and grounds behind the alternator,as well as battery cables and grounds; all of them.
Please let me know what you find.
Sorry to hear of your troubles.
As you have already replaced all the normal stuff and cleaned the ISV and its connector, I'd venture that this might be electrical in nature. By all means, I'd change the fuel filter just in case that you happened to have received a contaminated load of fuel that has partially plugged the thing up.
My best guess is two things:
1) Faulty/ intermittent DME relay. This affects the as well as fuel pressure,...
2) Battery/Alternator issue. Any variations in the operating voltage of the car will make for symptoms like these (and worse). I'd check that harness and grounds behind the alternator,as well as battery cables and grounds; all of them.
Please let me know what you find.
06-19-2002 | 08:08 PM
#4
Addict
Rennlist Member
Rennlist Member
Joined: Jun 2001
Posts: 2,537
Likes: 0
From: Dana Point, CA
How 'bout the distributor belt? Did you check it while you had the caps off? When mine failed back in '97 or so, it manifested itself mainly as low-RPM sputtering/break-up, although the engine ran rather sluggish in general.
06-19-2002 | 10:50 PM
#5
Thread Starter
Racer
Joined: May 2002
Posts: 251
Likes: 0
From: Tampa, Florida
Thank you for the leads. I will investigate further tomorrow. Tom's description sounds really close to what I'm getting. I will check as much of the wiring harness as I can check, though I'm a later MY95 build (with updated harness part number) EJ says that does not make me immune to the problem.
I did not check the distributor belt while changing the caps. Will check that as well.
Fortunately I'm going to a PCA tech session at SunCoast (Rennlist Sponsor) in Sarasota on Saturday so maybe I can get it on the lift and get some added advice.
Thanks to <a href="http://www.p-car.com" target="_blank">www.p-car.com</a> for the following DIYs which I used over a 17 hour period Monday and Tuesday (Plug Wires, Plugs, Muffler Change, Caps and Rotors, Brakes Pads) and Steve's tech statement on bleeding.
Will update and appreciate any more comments on things to check. This problem is pretty dramatic and happens pretty much every time I drive... instigated by really backing off the throttle to close to no gas. We'll see.
Thanks,
I did not check the distributor belt while changing the caps. Will check that as well.
Fortunately I'm going to a PCA tech session at SunCoast (Rennlist Sponsor) in Sarasota on Saturday so maybe I can get it on the lift and get some added advice.
Thanks to <a href="http://www.p-car.com" target="_blank">www.p-car.com</a> for the following DIYs which I used over a 17 hour period Monday and Tuesday (Plug Wires, Plugs, Muffler Change, Caps and Rotors, Brakes Pads) and Steve's tech statement on bleeding.
Will update and appreciate any more comments on things to check. This problem is pretty dramatic and happens pretty much every time I drive... instigated by really backing off the throttle to close to no gas. We'll see.
Thanks,
