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Part throttle hesitation and cold start stall

J

jcorallo

New member
Hi,

1997 Boxster 2.5 (UK).

1) Starts up OK from cold.
2) As soon as I try to move off by releasing the clutch, the RPM's drop and the car alomost stalls. I then have to apply more revs in order to keep the engine going. Doesnt do this when warm - I can get the car moving with less throttle/revs.
3) At part throttle, there is a slight hesitation, like the power is coming in and out very slightly.

Anyone got any ideas?

I've had it on the OBD2 ECU reader and no DTC's.

Cheers,

Jules
 
Hi Jules,

Me again. I had something reasonably similar. It was the air mass sensor that was on the way out on my old '99 986, gone but not forgotten ;-). If it is, it will get worse / most noticable as you move away from traffic lights etc. push the throttle and you get v little, declutch and feather the gas, try again and it works (just!!). NOT recommended for prolonged periods as you'll knacker the clutch. Its an easy fix. old out, new in. BTW, it didn't read as a fault on the ECU!

cheers,

Iain
 
Hi Iain,

Sigh - yes I did suspect the MAF. I'll maybe disconnect it and see how it runs which will point to this even more.

I've got some Isopropanol (electronics cleaner) so I'll wash it in this before I lash out the few hundred quid on a new sensor! [:mad:]

Cheers,

Jules
 
If you finally discover what the problem is let me know, I have had the same problem for 3 years.
Only when very cold i start up - fine, if i open the throttle just a little to set the revs between 1400 and 1800 the revs will suddenly drop as though you have turned off, it never stalls, goes back to tickover revs then immediately gains again - so i get this fluctuation, after 30 seconds of running all is perfect again.
Mine has been serviced, plugs changed, MAF changed, throttle body taken off and cleaned, the idle spped valve cleaned and i lubricated the throttle potentiometer, for a short while i thought i had cracked it - but we did not have very cold weather - now we have it is doing it again.
I now suspect it is the throttle potentiometer - The 2.5 has a cable opperated throttle, i intend to order one as i dont see this part as being expensive and it is easy to fit.
I too have no fault codes showing and i posted this fault on various Porsche forums without reply.

Glyn
 
Hi Glyn,

Well I've only just got the car at the start of this year, and I'm steadily working through the list of things to do. Thankfully I have my own laptop based OBD2 scanner tool. I'm using my HEX-COM (aka VAG-COM) - my other car is a VW Golf VR6 and I've lots of experience with that engine and similar sorts of problems and cures. The scanner tool does generic OBD2 so it works OK on the Boxster, thankfully.

The first thing I found when I bought the car (using the OBD2 scanner) was that one of the Lambda probes was faulty - one bank of cylinders was open-loop and the other closed-loop. So I've already replaced both (as a precaution) and also replaced all four of the exhaust clamps and one of the 3-stud flange gaskets (drivers side). I've yet to do the passenger side gasket because the drivers side one was such a pain!

I'll be hooking it back up to the scanner to check the short and long term fuel trims and checking for air leaks. It seems the air-oil-separater bellows are prone to failure and I've bought one of these ready to install just in case.

I havent even had time to look into the air cleaner and MAF area yet. I suspect its full of oil which may have caused the MAF to die. Not sure yet, but I will get to the bottom of it shortly - just a matter of time.[;)]

Jules
 
i suspect the throtle potentiometer as i squirted a bit of wd40 up inside the rotating centre shaft, even though the unit is sealed some WD must have entered the part via the rotating centre, i figure the graphite ring or the contact must be worn, now 10 years old (will be in April) and figure that most people would set the revs ready to pull away at about the same amount of throttle therby the graphite ring would receive more wear, again i suspect the throttle opens allowing air into the manifold and the worn ring fails to send a message to the ECU to tell it to inject more fuel - therby creating a weak mixture.
When really cold a weak mixture will substantially affect the engine.
When the ECU then detects a potential stall condition it opens the idle speed control valve and at the same time richens the mixture - onlt to then repeat a few times.
I could swear the fault temporarily went away after the WD, and WD does dry up quite quickly which would explain why the problem came back.

This is all just a theory at the moment but your posting has spurned me on to put it right even though i am now activly looking for an S.

BTW where did you get the OBD scanner - i would like one

Glyn
 
I told you that you had spurned me on so today i have ordered a throttle position sensor from Porsche-Apart, if this rectifies the fault - which i am convinced it will - i will let you know.

Mind you i have been convinced that everything i have done to correct the problem is "the thing"
My 97 will soon be a 2004 at the rate i am replacing things


Glyn
 
Hi Glyn,

The OBD scanner I'm using is VW specific - its called HEX-COM (aka VAG-COM from http://www.ross-tech.com/) but it does generic OBD2 (i.e. engine emissions related diagnostics e.g. lambda probes, temp sensors, mass air flow, throttle body voltage/opening percentage etc etc etc) thats why I can read my Porsche. But I wouldnt advise you get one of these since you cannot query the Porsche specific modules in the car, e.g. air-bag, ABS etc etc.

If I were buying one for the Porsche, I'd prob go for this one:

http://www.durametric.com/

Costs the same as I paid for my HEX-COM ~$250. With delivery and import duty and VAT, you'll be looking at about £250 all in delivered from the States.

If I ever need to diagnose ABS, air-bag etc, I'll be getting one of these for myself. Never used one on the Porsche, but looks to be the right thing.

Cheers,

Julian
 
The Durametric system is by far the best Porsche-specific DIY diagnostic software. The limitation with it is that it will only work on the first three cars that you diagnose. It reads the VIN and records it. The professional kit, with unlimited use, is twice the price. Understandable from their point-of-view, but it means you can only help out a couple of mates.
 
Forgive me for taking this rather technical thread down to basic levels, but is this not an inherent chraracteristic of the 2.5 Boxster when cold?

I had read a thread somewhere (on a different board) and also I believe the Boxster Owners Manual also states that when cold it has to be revved a little harder than normal to get it going until it reaches a warmer temp.

Mine's always had to have been revved a little harder on start up, but it hasn't caused me any concern.

Do you think this is interrelated to your problem?
 
Chrono, no engine should have the revs rise and fall without reason, my problem is so minor that i have been able to ignore it for so long, however now i intend to correct it -- It is a fault, not a characteristic of the Boxer engine.
When mine is cold and taking it out of the garage i simply let the clutch out - no additional revs at all - i dont have to rev it, it pulls away fine out of the garage and up the driveway, my driveway rises quite a bit but still no additional revs required.
Anyway the part is ordered and should be with me Saturday, it will be fitted within an hour of receiving and as i have not used the car for the last 24 hours and the weather is nice and very cold i will be able to see if the fault is rectified.
My original trade is mechanic - in the days when we repaired faults not simply bolted new bits on until the problem was cured or used a computer to determine faults - The throttle sensor is a sealed unit so i cant fix that, but without the aid of an OBD scanner i have simply looked at the problem logically from a mechanical engineering viewpoint, Im absolutly sure my problem is this part - but i will let you all know - and truthfully if i am right

Glyn
 
Hi

I also have this problem on my 1999 2.5 since I bought it with only 18,000 on the clock. I would suggest it is not the potentiometer as my car has so few miles on it , and secondly this would show up even when warm with mild hesitation - though I may be wrong. My view on this is that it may be due to some form of cold fuel enrichment / air bypass valve/ engine temperature sensor issue when engine cold. I would suggest maybe when started from cold and at idling the cold enrichment works fine and an air bypass valve passes more air to the engine bypassing the throttle. However in the transition to high revs, the fuel mixture is weakening - hence the stall, then as the revs increase the normal fuel/ air ratio is sufficient to keep engine from stalling. Possible that the cold enrichment does not continue sufficiently up rev range or temp sender to cold enrichment not working ??

I actually believe it is either a software / design issue - although I have not checked the MAF. For now I start engine and once idling settled, gently take revs to 2000rpm (fighting the tendency to stall) and hold the revs there for about 30sec to a min. This seems to work and I then set off without kangarooing down the road if I hadn't done the routine !
 
Goofy, we will soon know when i get the part.

I have changed the engine temperature sender unit, the 2.5 has a dual core, one signal to the ECU the second to the gauge, changed it and no difference.
Your car has very low miles and therefore your assertions may be true, what i dont know is how your miles have been put on the clock, If all 18000 miles have been done with trips to the shops or mainly in traffic then i would guess it is possible you have the same amount of potential wear of the same component.
Well the post man has not been yet - when he does it will be fitted immediately.

Glyn
 
Well - it sounds like we are all singling from the same hymn sheet.

What Chrono is describing is a weaker engine where you need to rev it more - the reduction in revs is caused by the intertia of the car when letting the clutch out. This is normal, and can sometimes be exaggerated with a cold engine.

What we are describing is the engine seemingly cutting out suddenly as soon as letting the clutch out when cold from standstill, the engine then regaining revs and bouncing about.

I actually forgot to mention the kangarooing when cold (thanks Goofy) which makes it hard to use just the throttle to control the car speed low down when cold. I tend to slip the clutch and get it up to speed then into 2nd gear etc. I HATE slow moving traffic queues because I cannot control the car smoothly at low speeds that well with the lumpiness of the engine.

As Glyn says, this is not a characteristic of any engine, and its deffinately a fault. I could not imagine taking my car for a test drive when new in 1997 and being told this drivability characteristic was normal Porsche. I would have laughed at the salesman and chucked the keys back at him.

Cheers,

Jules
 
I'm sure I'm right in remembering that, a few years ago, Porsche issued an update to the ECU mapping - this was to be "installed" at the dealers using the PST2 equipment.
I can't remember the exact thing it was intended to fix - but perhaps this is part of it and these cars have not been updated?

It was Chrono's post that reminded me of this...
 
Well I cleaned the MAF - no joy. So on to the next thing.

I remember reading about this update to the ECU - and that it was only applied to certain cars if the customer brought it to the dealers attention. I'll try and find the link to the article.

Anyway, I was experimenting with the revs today seeing if I could pin it down, to explain it better...

With the car in neutral, if I try and increase the revs as slowly as possible from idle, as soon as the engine reaches spot on 1400rpm, it misses a beat, splutters and drop to 1200rpm. If I then increase throttle slowly, the engine then sounds like it starting to misfire (but I dont get a check engine light) until it gets back to 1400rpm.

Sigh.....

Jules
 
Just as a matter of interest (ie I don't know if it applies to your problem) when I first got my 996 I had an American EZ-Scan 5000 hand scanner. It didn't show any faults on the car, but a PST2 test did reveal Porsche faults 27 & 35 (P1123 & P1125) in the DME. P1123&5 will not cause a Check Engine Light fault on RoW cars (only US) and I put it down to maybe the non-CEL faults are stored in a different register which my hand scanner couldn't access. In my case cleaning the MAF didn't get rid of the faults, but a new MAF did. I also have to say that I didn't notice much other than a slightly lumpy idle (when hot) which went away after fitting the new MAF. If you know someone with a PST2, it might be worth a second check.

I had a quick look through the Boxster TSB's, but I couldn't see anything about reprogramming. I'll try digging deeper though.
 
fitted the new potentiometer and although the engine was very cold it didnt seem to drop revs so dramatically, though it did drop, I am going to disconnect the battery to zero the ECU and see if this make the difference If not then the fault remains, dam and blast, i was sure i was right.
Anyway may not be a problem for me in the future as it looks like i will do the deal on an 02 S on Sunday, shame in some ways because the 2.5 has been pretty bullet proof - sure lots of niggly little faults but i have put them all right except for this.
Will let you all know how the battery disconnect goes and of course if i get the S i can look forward to a shed load more niggly faults that the previous owner has ignored.


Glyn
 
Glyn, and anyone else who has the problem,

What year and chassis number is your car?

Mine is early - 1997 and number 1428 off the production line. So mine must be riddled with bugs....

Jules
 
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