"Lurching" under light throttle
Thread Starter
Advanced
Joined: Mar 2009
Posts: 60
Likes: 1
From: Memphis TN
"Lurching" under light throttle
My '87 "lurches" under light or no throttle, like when coasting down in gear or cruising at 1500-1700 rpm or so. It's a rhythmic lurch or surge, strong enough to make your head bob. Step on the clutch or add a bit of throttle and the prob goes away instantly. Car runs great otherwise, and readings on vacuum gauge are normal.
Suggestions? The car is a Callaway, but I wouldn't think that would matter, this seems like an L98 problem. The car has 28,000 miles and is totally stock. No codes.
Thanks
Suggestions? The car is a Callaway, but I wouldn't think that would matter, this seems like an L98 problem. The car has 28,000 miles and is totally stock. No codes.
Thanks
Last edited by tiger38117; Apr 16, 2016 at 01:06 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Not likely a spark plug, under decel.
Does it seem like it's going in/out of DFCO (Deceleration Fuel Cut Off)? Once you let off the throttle at most anything above ~1200 RPM, as the car decelerates, if you pay attention, you should feel it coast for a moment, then it chops all fuel and the engine really drags the car down in a more pronounced way. Once that happens, the engine is essentially shut off, until it hits about 1200 RPM or so and the injectors come back on. So plugs, wire, EGR, timing...no engine controls should matter in this mode of operation -the engine should essentially be "off".
Can you tell if it's doing this? If not, I'd investigate why...if it is, then going in/out of that mode, I'd be investigating that.
Can you tell if it's doing this? If not, I'd investigate why...if it is, then going in/out of that mode, I'd be investigating that.
Oil Producer
Joined: Apr 2014
Posts: 9,372
Likes: 2,736
could a screwy tps set this off? set too high at zero throttle?
or a pulsing/figity iac?
what gets deactivated when you stip slightly on the clutch?
not tranny related? i was thinking torque converter lockup having a small seizure at first but then i figured out you were a manual.
or a pulsing/figity iac?
what gets deactivated when you stip slightly on the clutch?
not tranny related? i was thinking torque converter lockup having a small seizure at first but then i figured out you were a manual.
Not likely a spark plug, under decel.
Does it seem like it's going in/out of DFCO (Deceleration Fuel Cut Off)? Once you let off the throttle at most anything above ~1200 RPM, as the car decelerates, if you pay attention, you should feel it coast for a moment, then it chops all fuel and the engine really drags the car down in a more pronounced way. Once that happens, the engine is essentially shut off, until it hits about 1200 RPM or so and the injectors come back on. So plugs, wire, EGR, timing...no engine controls should matter in this mode of operation -the engine should essentially be "off".
Can you tell if it's doing this? If not, I'd investigate why...if it is, then going in/out of that mode, I'd be investigating that.
Does it seem like it's going in/out of DFCO (Deceleration Fuel Cut Off)? Once you let off the throttle at most anything above ~1200 RPM, as the car decelerates, if you pay attention, you should feel it coast for a moment, then it chops all fuel and the engine really drags the car down in a more pronounced way. Once that happens, the engine is essentially shut off, until it hits about 1200 RPM or so and the injectors come back on. So plugs, wire, EGR, timing...no engine controls should matter in this mode of operation -the engine should essentially be "off".
Can you tell if it's doing this? If not, I'd investigate why...if it is, then going in/out of that mode, I'd be investigating that.
Thread Starter
Advanced
Joined: Mar 2009
Posts: 60
Likes: 1
From: Memphis TN
could a screwy tps set this off? set too high at zero throttle?
or a pulsing/figity iac?
what gets deactivated when you stip slightly on the clutch?
not tranny related? i was thinking torque converter lockup having a small seizure at first but then i figured out you were a manual.
or a pulsing/figity iac?
what gets deactivated when you stip slightly on the clutch?
not tranny related? i was thinking torque converter lockup having a small seizure at first but then i figured out you were a manual.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
OK...that is a totally different operating mode that what you said originally;
"no throttle, like when coasting down in gear or cruising at 1500-1700 rpm" ....is very different than rolling through a parking lot in 4th gear. I think I understand you a little better now.
Typically surging under those conditions can be caused by:
*Lean A/F ratio
*Overly advance ignition timing
*Leaking/open EGR
*erratic sensor outputs
"no throttle, like when coasting down in gear or cruising at 1500-1700 rpm" ....is very different than rolling through a parking lot in 4th gear. I think I understand you a little better now.
Typically surging under those conditions can be caused by:
*Lean A/F ratio
*Overly advance ignition timing
*Leaking/open EGR
*erratic sensor outputs
Oil Producer
Joined: Apr 2014
Posts: 9,372
Likes: 2,736
op did u track it down?
i suddenly started to get it in my 87 as u described.
OK...that is a totally different operating mode that what you said originally;
"no throttle, like when coasting down in gear or cruising at 1500-1700 rpm" ....is very different than rolling through a parking lot in 4th gear. I think I understand you a little better now.
Typically surging under those conditions can be caused by:
*Lean A/F ratio
*Overly advance ignition timing
*Leaking/open EGR
*erratic sensor outputs
"no throttle, like when coasting down in gear or cruising at 1500-1700 rpm" ....is very different than rolling through a parking lot in 4th gear. I think I understand you a little better now.
Typically surging under those conditions can be caused by:
*Lean A/F ratio
*Overly advance ignition timing
*Leaking/open EGR
*erratic sensor outputs
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