Menu toggle

Turbo misfire

S

sulzeruk

Active member
Evening all, a guy has just bought an 1990 Turbo and is having a bit of trouble with it. Under hard load, he is getting a misfire at highish rpm at about 1 bar of boost. Car still has the cycling valve, manual boost controller and a MAF fitted. Has a single port wastegate. Sounds like it could be overboosting, cycling valve duff????
Alasdair
 
Alasdair If the car is running slightly more than standard boost and is ~ 20 years old, the chances are it could be the fuel supply = misfiring/cutting out under heavy boost. This is usually (in my personal experience) a sign of a failing fuel pump. Best bet, drain the tank, remove pump and purolator in tank filter. Clean and replace filter and reseal install a new pump. This worked for my car following issues at Silverstone on a GP track evening. Yours Chris
 
Fuel filter is outside tank visible in offside rear wheel arch Assume all fittings are rusty and pipes and filter will need replacement Also could be spark plugs or coil breaking down and rotor arm is also a key suspect. They also have a soft cut if over revved.
 
If it has standard chips then it will see 1 bar as overboost. If it has a manual boost controller fitted the the cycling valve has probably been bypassed (however the ecu will still reduce the ignition advance). Without seeing the plumbing its hard to diagnose but I'd start by reducing the boost to 0.7 & try it again.
 
Paul sorry to publicly disagree with you, but.... On the Turbo model (probably all models but can only speak from personal experience) there are TWO filters. The conical purolator that screws into the plastic fuel tank and the visible external aluminium cannister filter. When I replaced the fuel pump I had the whole system apart including the internal fuel filter. Yours Chris
 
Oh and just to prove I'm no going senile: Part 28 = 928.201.081.04 [link=http://dcauto.gotdns.com/illustration/index/2566601]http://dcauto.gotdns.com/illustration/index/2566601[/link] This looks to me as though its common to all 944's and probably 968's. The part number would also indicate it's used on 928's ?
 
All the usual basic ignition and fuelling stuf needs to be checked as sound, but apart from that I share Bertelli's view that it sounds like a funny configuration. Two questions spring to mind: 1) why is it running 1 bar of boost if that's all that's been modified? 2) what chips does it have in it?
 
I am quite competent with turbo plumbing mate after having installed a Tial DPW myself. Give us a shout if you need fella.
 
There is no way that the cv, an mbc and the single port wastegate can work properly together. If it really does have this combination, at 1 bar, it is obviously wrong so there's no point looking further straight away (eg draining the tank etc) - it's most likely the dme airflow based overboost protection cutting in as it sees too high an AFM voltage. (yes I know you said MAF but the DME won't know that). As has already been said, you need to find out what chips it has, how the MAF has been implemented and tuned and check exactly how the plumbing has been configured. I presume the misfire is real violent - like hitting the rev limiter? Initially, turning the boost back down would seem the smart thing to do...... If it actually doesn't have this combination, then plugs/gaps would be my first suggestion, followed by coil, leads, cap, rotor arm. Then pump & filter. Let us know how it goes?
 
I second that mate. Al the MBC as it is called is simply a pressure regulator unless it is a boost enhancer (which i am not too familiar with) in which case it still shouldn't be used with standard chips. You can set the MBC on a bench. All you need is a regulated air supply, which I believe you have. Set your air supply to 0.8 bar then feed it to the input of the regulator. Adjust the regulator until it just starts passing air. That should get you back to stock boost levels and in my opinion is a good place to start any diagnosis.
 
Thanks for all the suggestions guys, I thought the combo was a bit odd, will let you know how I get on, Alasdair
 
ORIGINAL: robwright I second that mate.  Al the MBC as it is called is simply a pressure regulator unless it is a boost enhancer (which i am not too familiar with) in which case it still shouldn't be used with standard chips.
Click to expand...
The boost enhancer (BE) is almost the opposite to an MBC. As you say, an MBC is simply a pressure regulator so it passes full air pressure straight through up to it's set point after which it mimits the amount of air through to this set maximum pressure. This can therefore be used with a dual port wastegate to supplement the wastegate spring and set a higher boost level. A boost enhancer lets NO air through until the pressure reaches the set point after which it lets the full air pressure through. So the BE is supposed to be used to block the signal to the standard wastegate, thus preventing it gradually opening and enhancing the build up of boost. Setting the BE higher than standard will result in a boost spike and then fluctuating boost. Of course, if the WG spring is weak the BE will be less effective as the exhaust gasses themselves will push the wastegate open anyway by acting directly on the back of the valve. Hope that helps?
 
I presume the misfire is real violent - like hitting the rev limiter? Initially, turning the boost back down would seem the smart thing to do...... If it actually doesn't have this combination, then plugs/gaps would be my first suggestion, followed by coil, leads, cap, rotor arm. Then pump & filter.
Click to expand...
The soft cut is a misfire - Overboosting is like putting the brakes on. Is it a misfire or like turning the engine off at full chat? If its a misfire on the red line its the soft cut if its lower but on boost I`ll wager the plugs but as above it all needs to be top notch.
 
As per the other thread, turned out to be the pressure pipe for the KLR ECU was crimped after an AFM gauage had been installed, I wonder if that was why the car had been sold??? Alasdair
 
The car will run fine with the KLR line disconnected assuming there is some external boost control such as MBC or EBC. Was the pressure line to the KLR completely blocked?
 
It was bent at about 90 degrees so probably. I disconnected it from the banjo on the KLR and it was a struggle getting up from the back of the mounting cradle for the ecus, Alasdair
 
By AFM gauge do you mean Boost gauge? Does the car still missfire with the boost line installed correctly? As the KLR is blocked and the boost wasn't going crazy I'm guessing boost control is via a single port wastegate and manual boost controller. It's probably just bleeding off a bit of the boost pressure in the line to the wastegate to regulate boost. Can you trace the pressure line from the top of the wastegate?
 
You must log in or register to reply here.

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Porsche
Lockton
Pirelli
Officially approved
Oracle Finance
WEX Esso
Collecting Cars
Back
Top