Acceleration Drop Off Question

[cparker944]

I've had this happen to me, from time to time, over the past few years but never knew the actual reason for this problem...

Yesterday I ran my 951 pretty hard, accelerating from a stand still to upwards
of 90-100 mph. The first attempt to accelerate was going well until about
70 mphish and then the car started to jerk violently until I let up off the pedal...
I tried it again and it began to jerk again...The third time it accelerated much better. I always speculated that I either needed a new fuel filter, cleaned fuel injectors, better gas or it was a timing issue... I'm sure many of you have experienced this in the past... What the h*ll is the cause of this issue?

Thanks for the feedback!

Craig

[Spidey944]

Have you checked your TPS switch? Unlike modern cars, our TPS's are simply an on/off switch. It has 3 switches. Idle, part throttle, and full throttle. Using a meter and www.clarks-garage.com test your TPS.

[cparker944]

Thanks for pointing me towards Clarks Garage. I'll research the TPS situation.
This info below, from Clarks Garage, sounds like Overboost might be the source of my
problem:

6.4 Turbo Overboosts
Problem statement: By far the biggest problem (at least so it seams) with my new 951 is that it cuts the fuel VERY abruptly during heavy acceleration. It is almost like someone is hitting the brakes! It usually happens around 4000rpm but will not occur if I just accelerate "normally". Also, my boost gauge is stuck at one when the engine is on (it returns to 0 when engine is turned off). I believe I can hear the turbo spinning during the acceleration so somehow the boost gauge must be connected wrong?

Second Problem statement: The car idles and runs fine, as long as you don't get on the boost hard. If you floor it, the boost gauge goes up to about 1.8, then, a couple of seconds later, the car jerks hard, and the boost goes away (fuel shut off??). I believe that either the cycling valve or the wastegate isn't working. Is there any way to check out which of these 2 items isn't working? Does anyone know of better designed parts to replace these?

Diagnosis 1: This is an overboost situation. One of the most common causes of overboost is the pressure line leading from the wastegate. A crack here will cause overboost for sure the DME then shuts off the fuel. This hose is rubber & can dry rot over time.

Diagnosis 2: You have a broken vacuum/boost line (hose) between the intake manifold between cylinders 3 and 4 and the KLR (Turbo control computer) box in the passenger footwell, or your KLR box is broken. If this hose is somehow cut or is leaking, the KLR will be reading atmospheric pressure, which will result in no real power for you. The KLR doesn't know how much boost, so goes into overboost protection mode (which is the abrupt cutoff of boost.)

Diagnosis 3: One possibility is that the connection to your air flow meter needs to be cleaned. The next is your wastegate is stuck. The next is that you have a leak in your lines going to the wastegate so that it doesn't measure the proper boost from the turbo and keeps letting boost build. Result: your DME/KLR will shut down the fuel so you don't blow your head gasket and you fly through the windshield. Inspect your hoses to and from the wastegate and cycling valve.

[nize]

i have never heard of the stock dme/klr shutting off fuel on overboost.

in fact, in an overboost situation, cutting fuel can cause the afr to quickly go dangerously lean.

[nize]

cparker944; can you list all of your mods for better diagnosis? do you have LBE? boost control? which chip? etc. etc.

[cparker944]

Thanks Nize....(Interesting) The "problem statements" explain exactly what my car is doing.

[cparker944]

Hey Nize...I bought my 951 from a guy who raced it locally. He definitely modified it's performance, but unfortunately I can't give you any specifics about the chip, etc. I have no LBE or Boost Control. Sorry...

[nize]

could it be from knock? bad gas? too much boost? do you have an aftermarket boost gauge? do you know exactly what boost level you're running?

[cparker944]

I don't have an aftermarket boost gauge. It's kinda hard to look down
at the gauge when this jerking happens because it's like you're riding
a bull and you just hold onto the wheel for dear life and watch the road...

If you've never felt this than it hard to describe except to say that it
is a very violent jerking motion and it does feel like the brakes are being
applied when you are traveling at 80 mph. When this happened on
Sunday, the fuel was cut off from the engine and the engine stalled...
Which scared me because I was thinking that the timing belt went
(Which happened to me years ago) or something of the sort. But, the
car started right back up again. Once again this statement is right on
for what I'm experiencing..."Overboost".

Problem statement: By far the biggest problem (at least so it seams) with my new 951 is that it cuts the fuel VERY abruptly during heavy acceleration. It is almost like someone is hitting the brakes! It usually happens around 4000rpm but will not occur if I just accelerate "normally". Also, my boost gauge is stuck at one when the engine is on (it returns to 0 when engine is turned off). I believe I can hear the turbo spinning during the acceleration so somehow the boost gauge must be connected wrong?

Second Problem statement: The car idles and runs fine, as long as you don't get on the boost hard. If you floor it, the boost gauge goes up to about 1.8, then, a couple of seconds later, the car jerks hard, and the boost goes away (fuel shut off??). I believe that either the cycling valve or the wastegate isn't working. Is there any way to check out which of these 2 items isn't working? Does anyone know of better designed parts to replace these?

[nize]

check the wastegate control line to see if it has been cut/disconnected.

if the cycle valve (stock factory boost controller) fails, it will result in an underboost situation and not overboost. overboosting shows the stock boost control has probably been bypassed, removed, or disconnected.

[smudo]

Today I experienced the exact same power cut of in 4th and 5th gears while accelerating hard. Everything is stock, except exhaust and Tial 38 mm waste gate with 3 bar spring. I did not experience the power cut-off in 2nd or 3rd gear. Could it be that the boost pressure that is building up in higher gears is bigger and the DME shuts for a second the fuel causing the power cut-off? Based on the above comments I think think it might be leaking pressure line leading from the waste gate, however, with 3 bar spring it normally should not build up to much boost?

I know that electronic boos controller would help to deal with this, however, I am not planning major power increasing modifications this winter. I will "open" the engine and inspect it and deal with worn down parts if required and instal Rouge A tune, and instal aftermarket AEM AFR gauge to monitor the Air Fuel ratio after chip tuning.

[DannoXYZ]

3-bar spring = 29psi boost
Are you really that much?
Lower it to exactly stock 10psi level and see if the stumbling problem still occurs.

[smudo]

Originally Posted by DannoXYZ

3-bar spring = 29psi boost
Are you really that much?
Lower it to exactly stock 10psi level and see if the stumbling problem still occurs.

Hi, did not notice your comment. Obviously I was wrong about the 3 bar spring I have the most weakest spring that comes with tial WG i.e. 0.3 bar that is only about 4.5 psi (according to the shop that installed the WG - I will double check it). Thus, I was surprised to have that power cut-off. It indicates that something is not right. Maybe the vacuum line to the WG is leaking. Have driven since then the car only in city, thus I have had the WOT till redline only in 2nd and in 3rd till maybe 5k RPM, but the stumbling did not occur. This weekend the car goes to workshop for suspension change and this would be good opportunity to deal with this issue. Any further suggestions are welcome.

[DannoXYZ]

there is a rubber hose somewhere that is coming loose under boost. Check the two thin connections at the intercooler. By check, I mean remove and inspect. those are very thin rubber and can split radially parallel to the hose clamp.

check the conical hose going into the throttle body.

remove and check the outlet hose at the turbo. Not unusual to have a split on the bottom that opens up under boost.

What happens when you develop enough boost to open up these hoses is a lot of air escapes. But the DME is still injecting the right amount t of fuel. Without the air that escaped, it'll be too rich and stumble.

[smudo]

Thanks, Danno. So you think that the stumble is due to to rich fuel/air mixture?

Is there any self protection mechanism in the stock DME that would cut off the fuel delivery in case certain boost level in the system (I mean the air intake pressure) is exceeded?

I have purchased AEM WIDEBAND AFR LSU 4.9 UEGO, but I will install it during the winter. f I had it on, I could see how rich the mixture is. At what rate generally would it start to stumble? 8:1?

[DannoXYZ]

Yes, the stock overboost protection in the DME kicks in at around 4.7-4.8v output from the AFM. Easy enough to hook up a voltmeter to the AFM input line at the DME-harness. It's a thin green/red-stripe wire I think. Then monitor AFM-voltage while you drive and see if the stumbling occurs in-sync with 4.7-4.8v AFM-voltage.

Very unlikely with 4.5-psi boost though. Most 951s at this age have weak wastegate springs and are running at 2-4psi boost also and they're not all of them are stumbling from hitting overboost-protection. So in your case, it's most likely a leak in the intake-tract after the turbo-outlet somewhere. Due to mechanical-nature of boost-control, you develop more boost in 4/5-gears. Thus the crack/leak in the hoses only opens up in those gears. If you were to adjust boost to stock 10psi levels, you'll most likely get this stumbling in all gears.

I've seen stumbling start around 9.5:1 AFR. At least it's intermittent in your case. Continually running rich will wash the oil off your cylinders and cause accelerated ring & cylinder wear. Use datalogging with your wideband rather than looking at a gauge. Much more useful data that can be examined at a later time... and safer too...

[notMyScreenName]

Danno...

I've long wondered, is the stock spring available/replaceable (for those trying to stay entirely original) or is it simply not worth it to keep the original WG?

[smudo]

Originally Posted by DannoXYZ

Very unlikely with 4.5-psi boost though.

I get the standard boost level with 0.3 bar spring in the 38mm tial (it was measured with aftermarket boost gouge). Also standard boost gauge that shows 0.1 bar less than actual boost level, shows that I get boost levels around 0.75 bar (10 Psi). With 0.6 - 0.8 bar springs the boost was over 15 Psi.

Will also try to put on the AFR gauge sooner to get more information what is happening with the engine.

I will look for air leaks after turbo outlet. Thanks for the advise.

[DannoXYZ]

Originally Posted by notMyScreenName

Danno...

I've long wondered, is the stock spring available/replaceable (for those trying to stay entirely original) or is it simply not worth it to keep the original WG?

Personally I really like the stock wastegate. The diaphragm on stock wastegate is spot-welded together. However, I do have a procedure that allows upgrading the spring. Also have procedure to convert to true dual-port where exhaust-pressure rather than intake-boost is used to clamp the valve shut. This is significant because exhaust-pressure goes up with RPM. Balancing exhaust-pressure on the valve's head by using exhaust-pressure to cancel itself out allows for simpler management of boost on the diaphragm side. Kinda like how the FPR works using manifold-pressure to maintain constant pressure-differential across the injector's inlet & outlet.

In either case, with upgraded stock wastegate, I've used electronic controllers to get 25psi to redline without any problems. Although I prefer a rising boost-curve with rpm and lower boost in lower gears for traction-control.

What a lot of people don't realize when upgrading to bigger turbos and more power, is that the wastegate vents less exhaust than stock. Some of the Honda guys use no wastegate at all and just tune for whatever boost the turbo can generate, typically 35-45psi in the mid-range that drops to 20 by redline.

[smudo]

Danno, I have thought about what you suggested and realized that before I changed the OE WG with tial WG, the turbine could build up maximum 0.55 bar (measured with aftermarket boost controller). Thus, if the stumbling would be due to too rich mixture, I would have experienced it before. I was two times on track and there were two 4th gear straights and I did not have the stumbling.

I did almost 70 miles on highway and did also longer distances with around 100 mph. With half throttle in 4th and 5th the stumbling did not occur as well it did not occur in WOT in 3rd.

Today I spoke to the mechanic who will upgrade my suspension (he has no experience with Porsche's and petrol turbo engines) and he said that maybe I do not get enough fuel in the 4th or 5th gear. I most probably have clogged fuel tank ventilation valve (I smell fuel in the carbine after each drive), that might also hinder the fuel delivery (due to the vacuum in the tank). He will try to measure the fuel pressure and clean the ventilation valve/filter. He will check also the vacuum lines.