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Hits wall at 4000 rpm/limited performance - ref sensors? http://forums.tipec.net/viewtopic.php?f=25&t=23443	Page 1 of 6

Author:	mikseymono [ Fri Jul 06, 2018 9:15 am ]
Post subject:	Hits wall at 4000 rpm/limited performance - ref sensors?
Hi team, First post here...be gentle.... So have had my 924S (160bhp) 1988 for a month now. Sorted out quite a few minor quirks but this one eludes me. Symptoms: Car starts and idles well. It will cruise at 70mph at 3100rpm...however, it is very slow in getting there and has limited performance. The revs in all gears seem to max out at around 4000rpm and it will not rev freely to the redline at all, standing or moving. It feels like it has hit a rev limiter. I have checked, as per Clarks and Haynes: TPS, Dizzy cap, rotor and leads, Vac leaks, Changed fuel filter and the lines immediatly before and after, changed sparks plugs and filter, new oil and filter, Fuel pressure test, Fuel flow test, Tested fuel flow pump, DME relays, DME computer, checked Belts (aligned BS pulley properly), AFM, Ignition coil, Engine compression test, cleaned as many earth points as I can on the car...so you can see quite exhaustive. I have read that the reference sensors could be something to do with it. I have tested as per Clarks, but not with an Oscilloscope and they come out on spec. I have different opinions in how to test them in situ...some say turn key to on (no crank) and the tacho should jump and others say crank engine and tacho should jump. Which one? I have already signed up to Pelican Parts (USA) and Porsche924 (UK) forums and now here! So I am putting this out to a wider audience in the hope that this is not a unique problem and someone has been there before! Open to all suggestions! Thanks, M

Author:	Waylander [ Fri Jul 06, 2018 9:30 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Tracks on the AFM? Fuel pressure regulator Slack throttle cable

Author:	mikseymono [ Sat Jul 07, 2018 7:54 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Tried all those. Still stumped................. M

Author:	Prophead [ Sat Jul 07, 2018 10:44 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Well that is a pretty extensive list and would sort out most running issues. You are basically either running out of spark or running out of fuel at the higher rpm range, seeing as you have been through the ignition system and air intake system, the only other suggestion I have is Fuel Injectors or fuel pressure regulator and or fuel damper.

Author:	mikseymono [ Sat Jul 07, 2018 10:52 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Thanks. Well, I have done the fuel pressure test at the rail and that was spot on as per Clarks. So on that basis I would have thought the FPR is OK. I have a hunch ( hope?!) it will be the reference sensors. Have ordered a pair and will have to wait. There are various threads on the web about these having issues. It does feel more electrical rather than fuel related if that makes sense. Cheers, M

Author:	Prophead [ Mon Jul 09, 2018 10:49 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Please post the outcome / fix, would be useful for future referenc. Hope you get it sorted.

Author:	mikseymono [ Mon Jul 09, 2018 10:59 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Of course Prophead! Sensors won't arrive for a other 6 days or so. I have ordered some more ignition leads as they looked in poor condition but don't think they would contribute massively to the issue I am having. Will let you know when I know! Thanks, M

Author:	Gryphon [ Thu Jul 12, 2018 3:52 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Probably not all that helpful, but from memory it's at around 4000 rpm that the AFM is wide open and the signal no longer used for fueling. It's also wide open at lower RPMs but higher throttle, where it sounds like your engine might also struggle? That could indicate that the AFM is good but whatever signals it uses to calculate fueling once the AFM is maxed out are not so good. Unfortunately I can't remember or work out what those are! I know engine speed, so reference sensor is a big one, but as the engine doesn't have throttle position senor or manifold pressure sensors, I can't quite work out how it would calculate the fueling. I could also just be talking bull... but I'm pretty certain that the AFM does max out at surprisingly low airflow, and it's supposed to run a bit rich above that for safety. Otherwise, if it's not the reference sensors I'd agree with Prophead and check injectors and spray patterns. Good luck

Author:	mikseymono [ Tue Jul 17, 2018 11:54 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
OK...so..managed to get sensors out...not too bad.....the one nearest the bulk head in hole 'D' was badly eroded on the top as in it was rubbing against metal. I gaped new sensor as per Clarks. I stuck an 8mm thick cut up credit card to gap them. I cranked the engine. Straight away the tacho jumped up to around 4500 on start with no foot on accelaration...first time I have ever seen that...exciting, engine felt much more alive.....however there was this high pitched metal grinding whine. Sure enough I checked new sensor and the new one was eroded on the top as well. It turns out the aftermarket one is slightly...maybe...1mm longer than the OEM. There does not seem to be the amount of play on the up down motion of the bracket on mine against this instruction vid https://www.youtube.com/watch?v=-MbghejYiqk I have now put the old one back on as I have no more! Convinced it is the speed sensor that is the problem. So question is, how hard is it to gap and if the sensor was scratched eroded would this make the difference? Was a bit adverse paying top whack for OEM Bosch sensor so bought these instead https://www.ebay.co.uk/itm/CRANKSHAFT-S ... 2749.l2649 Thoughts please. Thanks, M

Author:	mikseymono [ Wed Jul 18, 2018 2:29 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
Well, I changed the sensors and NO difference. I have done so much to my car and the only thing to have made any driving difference is lining up the balance shaft properly. I really thought/hoped it would be sensor related. I gapped to around 9mm and it is the same. It was just when I gapped too close..i.e on the flywheel, the revs really took off....so thought I was on to a winner there. For the moment I can not get it to rev past 4200 rpm. Open to ANY suggestions. Thanks, M

Author:	Gryphon [ Wed Jul 18, 2018 3:02 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
I did exactly the same with an aftermarket reference sensor... Flywheel damaged the metal casing but it seemed to work fine once I'd added a 1.4mm plastic washer to space it out. Where in the country are you? Might find someone willing to let you try their DME to eliminate that...

Author:

mikseymono [ Fri Jul 20, 2018 2:37 pm ]

Post subject:

Re: Hits wall at 4000 rpm/limited performance - ref sensors?

So, it seems I don't have the 0.3mm washer that goes between the sensor and the bracket as per the PET diagram. No 29. Is this really going to make any difference? I understand the gap should be 0.8mm from end of sensor to teeth of flywheel and imagine it is required to get the sensor sitting square to the flywheel.....but 0.3mm is really rather small for the washer that sits inbetween bracket and sensor.......am I on the right track?!

Attachments:

washer.jpg [ 15.11 KiB | Viewed 7737 times ]

Author:	Prophead [ Sat Jul 21, 2018 8:36 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
if you have your old sensor, worth checking it's not burried in crud and stuck to it. 0.3mm is a larger % of a 0.8mm clearance, it's all relative. Call your local OPC it'll probably be in stock somewhere and doubt its that expensive. It's always the small things that look un-important that end up being significant lol

Author:	mikseymono [ Sat Jul 21, 2018 8:41 pm ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
You are quite right it is all relative. Been thinking about it and all the angles. I have ordered 0.8mm washers, the 'S' 0.3mm washer and two more sensors. I hope i am on the right track....it feels close..m

Author:	mikseymono [ Mon Jul 30, 2018 8:37 am ]
Post subject:	Re: Hits wall at 4000 rpm/limited performance - ref sensors?
well...0.8mm washers have arrived but the sensors and the 's' washer have yet to find their way here................... The 0.8mm washer are......well.......really thin.....I can see (hope) how the 0.3mm 'S' washer could make a difference...will also put the sensor nearest the block 0.3mm closer as well.... Will keep you updated..... M

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