Jerkiness at very light throttle openings
Discussion
My '91 Eunos 1.6 is very difficult to drive smoothly when trickling along with the throttle just cracked open very slightly. It's particularly noticeable in crawling traffic when you really need to dip the clutch to avoid a jerk when just rolling off the accelerator but it's actually there at any speed, just less noticeable in a higher gear.
At first I thought I had a sticky throttle cable but I'm now fairly sure it's the engine management system shutting off the fuel supply too abruptly when it decides the engine is going into overrun mode and/or when it thinks it should be ticking over but is running too fast.
Does this sound like a possible explanation and does anyone know how to fix it? It's not a major problem but it is irritating!
(P.S. Took it down to the Goodwood Festival of Speed on Friday. The traffic was so much more tolerable when you could look up at the trees and clouds and generally enjoy the countryside!)
At first I thought I had a sticky throttle cable but I'm now fairly sure it's the engine management system shutting off the fuel supply too abruptly when it decides the engine is going into overrun mode and/or when it thinks it should be ticking over but is running too fast.
Does this sound like a possible explanation and does anyone know how to fix it? It's not a major problem but it is irritating!
(P.S. Took it down to the Goodwood Festival of Speed on Friday. The traffic was so much more tolerable when you could look up at the trees and clouds and generally enjoy the countryside!)
I'd make sure your throttle cable is set properly, and that the linkage is all free, then properly set the idle.
Other than that though - mines not particularly smooth either - thank god the clutch is light because it would be an abolsute b
h in traffic otherwise. The transition from idling along, to accelration, is a big jerk.
Other than that though - mines not particularly smooth either - thank god the clutch is light because it would be an abolsute b
h in traffic otherwise. The transition from idling along, to accelration, is a big jerk. 
Once properly warm she idles at 850ish but sometimes hovers at around 1100rpm for a few seconds before sinking down to 850. That said, she's smooth enough.
I'm pretty certain the throttle cable is fine and is properly adjusted.
An example of how it behaves is when in traffic at, say, 30mph in 4th with just a very light throttle opening. If you then ease back on the accelerator, going from very slight throttle to closed, you get a jerk, like someone just switched off the ignition although it's not that. Hence my suspicion that the engine management is cutting the fuel abruptly. Reapplying a smidgen of accelerator then produces another jerk as the engine picks up again. It's almost as if the electronics are working in binary: its either stop or go but without any graduation in between, but only for this very first part of the accelerator travel.
Like I said, it's not a huge problem but it is frustrating when you're just trying to flow smoothly.
I'm pretty certain the throttle cable is fine and is properly adjusted.
An example of how it behaves is when in traffic at, say, 30mph in 4th with just a very light throttle opening. If you then ease back on the accelerator, going from very slight throttle to closed, you get a jerk, like someone just switched off the ignition although it's not that. Hence my suspicion that the engine management is cutting the fuel abruptly. Reapplying a smidgen of accelerator then produces another jerk as the engine picks up again. It's almost as if the electronics are working in binary: its either stop or go but without any graduation in between, but only for this very first part of the accelerator travel.
Like I said, it's not a huge problem but it is frustrating when you're just trying to flow smoothly.
snotrag said:
What other cars are you used to?
Light car, peaky engine, prehistoric engine/fuelling management... Yours sounds about the same as mine.
MG Midget, Lotus Elan S2, Mini Cooper S with full race Downton 1293cc engine, Escort Twin Cam, Rover Vitesse, Impreza Turbo, plus the current company desiesel Astra!Light car, peaky engine, prehistoric engine/fuelling management... Yours sounds about the same as mine.
Actually, a pretty wide selection over the years. If I'd just bought the Eunos new and it was doing this I'd be back at the dealer's saying "sort it". I'm sure it didn't start life like this!
Actually, I've just thought of another clue: it only does it when fully warmed up. Presumably when it's cold the engine management is fuelling it richer so the problem doesn't occur.
MX-5 Lazza said:
It could be a dodgy throttle position sensor...
Just to update this one, it does indeed appear to be the throttle position sensor. After trying various things, I've adjusted the TPS as far as possible so that it's probably not registering a closed throttle at all and the jerkiness has vanished (but the tick over has gone up by about 250rpm). Next step is to set it up so that it's adjusted properly but for the moment it's great to be able to drive smoothly again. I think without really noticing it had become more irritating than I'd realised!Stickers said:
Could also be down to plugs or the infamous HT leads.
Not this time. Having now adjusted the TPS (the mounting holes are slotted to allow a fair bit of adjustment) she's now picking up the throttle and releasing it again as smoothly as a very smooth thing! Makes me feel like a chauffer compared to the previous jerky progress.MX-5 Lazza said:
Nope, there is nothing to adjust on a Mk2.5 TPS.
I could be wrong but I think the Mk1 might have a very simple TPS that just registers Closed, Open & WOT whereas the later cars had a more complex TPS that measures throttle opening %. Can anyone confirm this?
That's right. On the Mk1 my understanding is that it sends one signal for throttle closed and another at a different voltage for throttle wide open. No signal = anything in between. The manual says to set it up with a multimeter, which is reasonable, but I found that the point when it sends the throttle closed signal was detectable by a change in engine tick over speed, making it very simple to set up.I could be wrong but I think the Mk1 might have a very simple TPS that just registers Closed, Open & WOT whereas the later cars had a more complex TPS that measures throttle opening %. Can anyone confirm this?
On later cars, including 1.8s, the TPS sends a linear signal across the full range of throttle movement.
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