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brian j

Motronic 3.1 Jerky Acceleration

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brian j

I have been running motronic 3.1 from a zx volcane converted to delete the mixture pot and replace it with a lambda sensor for about a year now and I have always found it to be jerky when going from a trailing throttle to acceleration. There is a very noticeable pause where nothing happens for a fraction which feels like you have backed off the throttle and then it picks up and goes.

If I go from light acceleration to harder acceleration it is smooth as silk.

It also wasn't happy to idle at less than 1500 ram without stalling when dropping back to idle from 2500rpm or more.

 

I had always thought that this was due to me running a longman big valve head and piper pt70 cam so the map was well off for the engine spec. (I'm still saving for that one) but last week the throttle pot died and I swapped it out all good with the water and air Inlet temperature sensors and was rewarded with 2 days of it being beautiful to drive. No jerky on and off throttle, Just smooth and we'll behaved from cold right up to hot. It also idled better without stalling.

 

So I decided to take the 205 for a weekend away (my wife and I's 3rd anniversary) and it went back to its old behaviour within 20 miles or so.

 

Any ideas on why that should be so? I am wondering now if there is something other than just needing a remap so I am after opinions as to whether I am missing something?

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Anthony

My gut feeling is that the TPS - or something related to it - is the issue.

 

Reason being is that the TPS is there primarily to tell the ECU about changes in throttle opening, and in relevance to your symptoms, to allow the ECU to provide momentary enrichment when the throttle is opened whilst the MAP sensor (the ECU's main source for engine load and thus fuelling needs) catches up. With no TPS signal, the fuelling will likely go lean briefly when the throttle is opened, causing the lag/hesitation sensation you're experiencing.

 

I'd hazard a guess that with no valid reading from the TPS it would impact idle too.

 

That you replaced the TPS and the problem briefly went away makes me wonder if there's a bad connection or broken wire in the loom related to it - I don't have the MP3.1 ECU plug pin out to hand, but if you unplug the ECU and use a multimeter between the relevant pins you should see the value change in a smooth linear fashion (depending which two pins, it'll go either up or down) as you open the throttle. No change in reading, or a jerky/erratic change would point at a TPS issue.

 

I haven't tried it on MP3.1 and don't know what's "meant" to happen, but if you unplug the TPS does the fault change?

 

I should add that this is only a guess and not based on specific MP3.1 experience with this fault.

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welshpug

I've had pretty similar behaviour from leaky MAP pipes.

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brian j

Thanks guys!

 

I am with you on the tps Anthony. It seems to make sense. My replacement was an el-cheapo one from Neat autos so I wasn't 100% confident of it, but everywhere else sells intermotor kit for 3x the price and I've never had good luck with intermotor parts. I had played with the he old one on a meter and it didn't look too bad, but Googling suggested that wasn't always a good indicator and the new sensor working seemed to confirm that... I have replaced the JPT plug and crimped new terminals for the tps because they were a bit manky but that hasn't worked.

 

On the suggestion of the map pipe. I assume I could test it the same way as for a distributer advance diaphragm, by pulling the end off the Inlet and sucking it to see if it leaks?

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brian j

OK. I Pulled the map sensor pipe today and it isn't leaking.

I pulled the sensor wiring plug off and reseated it, and it is currently back to behaving beautifully. I'll keep an eye on it and if it happens again I may have to trace back the wiring and look for bad joints or damage that could cause issues if it is moved.

 

My real theory is that the 205 is jealous. It hasn't caused me real problems in 5 years apart from the twice that I have taken the Mrs away in it...

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