Hesitation under part throttle

[hartge bob]

Been out getting used to my new turbo and come across a peculiar problem.

On part throttle there seems to be a moments hesitation which sounds like the turbo is boosting then stalling then boosting again very quickly. The feeling is not unlike you sometimes used to get with a car on carbs when the chokes would take a second or two to spluttering before clearing. It sometimes clears itself after a few seconds but sometimes requires a lift followed by another bootful to get it going.

It is particularly noticeable on motorway / dual carriageway cruising where you are more likely to be on and off the throttle without actually flooring the throttle. The problem doesn't manifest if the throttle is floored under the same circumstances so I don't think it can be overboost protection cutting in. On full throttle there is just a gradual build up in speed until the boost builds up when the speed increases a bit quicker [] (boost enhancer on the way)

Has anyone else ever come across this or have any ideas?

Thanks
Rob

 

[944Turbo]

Not sure, but I would start by checking the vacuum lines for leaks loose conections, then check the plug leads (open the bonnet in the dark and check for sparks), then maybe have a look at the distributor cap rotor arm.
Could be a tricky one to pin down,

Tony

 

[richthomas]

I had, and am still having a similar problem with my S2, I had a bouncy idle, and surging on light throttle.

I replaced the throtlle switch and that has sorted the idle, but I still get a slight surge or kangaroo on light throttle. It's a bit like a very slight misfire. I think it could be down to the airflow meter on mine, as they get older the metallic surface of the pointometer inside gets worn away, and so sends the wrong information to the DME. This can make the engine run badly if you happen to keep the throttle on the worn part. That is why if you floor it it is OK, as I guess the middle part of the sweep gets worn more than the extremes as it is used more often.

I still haven't taken the plunge and got a new airflow meter yet, as I'm still unsure and at about £250 for a rebuilt one it's not really something I want to just get to try. It could turn out to be a very expensive way of trying to diagnose it[] They are supposed to be difficult to test so it might be the only way.[]

It might be worth you checking yours, or getting it checked as you never know it could be down to the same thing if you draw a blank looking foe vacuum leaks etc.

 

[Fen]

My cab does exactly the same as yours, Rich. I also believe it to be the AFM. It isn't that bad though (only really noticeable at 30mph in 3rd, which is something I do surprisingly little) so I'm living with it for a while longer yet.

 

[John Sims]

I still haven't taken the plunge and got a new airflow meter yet, as I'm still unsure and at about £250 for a rebuilt one it's not really something I want to just get to try.

Sounds like a good excuse to slap in a MAP kit to me []

 

[Diver944]

Rob, there are just so many electrical sensors and switches on the 944 Turbo that it could be many things. Easy things to try yourself are an injector cleaner in the fuel or a few tanks of good quality Optimax, check all the vacuum pipes as mentioned, and also clean all the electrical contacts you can lay your hands on under the bonnet. After that get a good specialist to test and clean things like the throttle switch, air flow meter, injectors.

When was the last time it was serviced and had new plugs, air filter, fuel filter etc? Is the throttle cable tight on the throttle cam?

I also had similar surging in my S2 a few years ago and had the AFM cleaned, that improved things greatly. I have to say that if my AFM was knackered I would definitely think about getting a modern MAP sensor to replace it instead of another restrictive AFM

 

[richthomas]

only really noticeable at 30mph in 3rd

That's what mines like at the mo, also some times at a steady 70 on the motorway, but I suppose it's about the same throttle position.

I had a look for MAF kits but they all seem to be for turbo's, and expensive too.

Paul, are you still coming to Autobahn tonight?

Rich

 

[wedgepilot]

ORIGINAL: Diver944
I also had similar surging in my S2 a few years ago and had the AFM cleaned

Paul, what's involved in cleaning the AFM? Is it cleaning up the electrics, or a 'physical' clean with something like carb cleaner? Or both? Is it a DIY job, or strictly 'specialist-only' territory? It sounds like something that would be worth doing after 15-odd years if hasn't been done already.

 

[richthomas]

I removed mine, cleaned out the barn door and insides with carb cleaner, but it didn't help. The electrical contacts are sealed in under a plastic cover and I didn't want to destroy it to get it off. That's the bit that would benefit most from a good clean.

 

[hartge bob]

Mine definitely feels (and sounds) more like the turbo itself is stalling briefly, but this may just be a symptom of something else.

Thanks for all the ideas, unfortunately I can't try any of them at the moment because it's up on axle stands while the wheels are being refurbed.

Soon as I get four wheels back on my wagon I will try the suggestions.

It has just had a service Paul, don't know if the plugs were done though.

Rob

 

[944Turbo]

ORIGINAL: richthomas

I removed mine, cleaned out the barn door and insides with carb cleaner, but it didn't help. The electrical contacts are sealed in under a plastic cover and I didn't want to destroy it to get it off. That's the bit that would benefit most from a good clean.

I took the top off mine (brute force mostly) and had a look at the tracks -cant remember why now - anyway there was no problem with it, stuck the top back with silicon,
Tony

 

[944Turbo]

Back to the turbo, I also had a similar problem (but without any turbo noises) when my coil was failing - there is a section in the factory manual somewhere that tells you what the resistance should be, (can be checked with a multi meter but once cables disconnected from the terminals), interestingly once I had the old one out I shook it and the values came back.
Tony

 

[Diver944]

ORIGINAL: wedgepilot

Paul, what's involved in cleaning the AFM?

Dunno [&:]

At that time I wasn't as brave at doing things myself as I am now, I just had it done when the car was serviced. From memory they charged about an hours labour, and said it was well gummed up with residue after about 130,000 miles of driving. I had the injectors ultrasonically cleaned the year before for similar reasons

 

[uktrucks]

Rob
Any progress yet ? It sounds as if some safety device is opening the wastegate as manifold pressure is rising, thus dumping out the pressure. Then allowing it to build at second attempt ? This in itself is confusing as to why it stops boosting then starts and goes to full boost.
Are you perhaps not just asking it for too much boost at too low revs ?

Allan

 

[hartge bob]

Allan,

I've not really had a chance to try much really yet as the wheels aren't going to be done until next week sometime so it's still up on axle stands at the moment.

I originally posted on here about this in case anyone else had come across it before and had a quick solution.

I didn't notice what the boost gauge was registering while it was happening, when I get back on the road I will try to pin down exactly the conditions it happens under and keep you posted.

I'll then try putting the boost enhancer on to see if this makes any difference.

Thanks
Rob