gatorbait

At partial boost, above 3500 RPM and with the throttle approximately half open, my 951 has taken to surging badly. The problem is less bothersome at full throttle, but it surges then, too. The problem does not exist off-boost, as the car accelerates smoothly.

The problem has existed to a lesser extent since I bought the car in December, but it got considerably worse when I replaced the fuel injectors on Monday in order to fix a warm-start problem (fortunately, the warm-start problem was resolved).

Any idea on what ails? I'm thinking it's the stock air flow meter, but I'm not sure. The motor seems otherwise strong and healthy. The car has been recently tuned, and has a new fuel pump, fuel filter, TPS switch, fuel injectors, and air intake. Even with the surging problem, the car's a bullet above 3500RPM.

Any advice would be appreciated.

Thanks

adrial

I notice you probably have a boost gauge because you set your reliaboost...do you also have an a/f gauge?

IF so, what does it do when you're feeling the surging?

gatorbait

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by adrial:
<strong>I notice you probably have a boost gauge because you set your reliaboost...do you also have an a/f gauge?

IF so, what does it do when you're feeling the surging?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Adrial, all I have is the stock boost gauge and I don't have an a/f meter, so all I can offer is a very unscientific seat-of-the-pants observation.

By surging, I mean that the boost comes on unevenly. The car lurches forward, pauses, and lurches forward again (in very rapid succesion). I had the a/f mixture adjusted recently by a Porsche shop that specializes in 944's (my car would be there now, were it not for the fact that he's closed the shop for two weeks as he's building a track car and sponsoring two drivers for an upcoming race). Again, the problem worsened when the new injectors went into the car.

No problem off-boost, less of a problem at full-boost.

adrial

Does anybody else think it could be detonation, the car pulling back timing, and then accelerating for a little bit, then repeating the cycle???

How did you dial in 15psi without a boost gauge?

gatorbait

Adrial, hadn't thought of timing. Maybe that's worth a look.

The Reliaboost-1 (which is similar to the Lindsey boost enhancer) came pre-set at 15psi. Last week, my mechanic, (using his gauge) could only get my motor to hold .95 bar (slightly less than 14psi), which means that the Reliaboost-1 probably gives an approximate value.

adrial

Did you receieve a printout of the a/f curves from the mechanic?

sm

Is the TPS adjusted properly? Does it "click" at the slightest opening of the throttle?

gatorbait

Don't have the A/F curve, but all is well with the TPS.

fast951

I think the problem is related to your fuel. Things appeared after you changed injectors. Have you checked your fuel pump? What fuel pressure are you running? Maybe the mixture is way off.
Another thing to check is the Air Flow Meter. Maybe there is a bad spot on the film inside the box. Borrow a good AFM and see if it helps.
Detonation is a possibility, but what's causing it?

Start checking the simple things first (Fuel Pressure, TPS, Temp Sensor, intercooler hoses, AFM). You'll learn plenty and you'll find the problem.

Good luck.

Peckster

Could you call it bucking rather than surging?

Mine surged gently under partial throttle when I first got it, but a number of changes made it go away gradually. I replaced a number of hoses, not all at the same time, plus I had my injectors cleaned.

There must be a few threads in the archives on this.

gatorbait

Fast951, I installed a Bosch 3-bar FPR in February, and a new fuel pump last week. A new TPS was installed last month (which cured the problem I had with the car backfiring every time I lifted off the throttle).

The surging problem has existed since I bought the car in December, but only got to be objectionable after the new fuel injectors were installed on Monday. Were it not for the surging though, the current set up would feel great.

The mixture was adjusted two weeks ago, which corrected a problem I had with it running too rich at idle through midrange (all too common with the APE chips that I have). I tend to think it's the airflow meter rather than detonation but I'll check the timing and the AFM. I'm hoping it's something simple.

Peckster, bucking would probably be descriptive of what I'm experiencing as well, but I've had all the hoses/lines checked in addition to all the other stuff I've done.

gatorbait

Almost forgot--I also replaced the blow-off valve in March. Again, no impact on surging throttle problem at partial boost.

RJP 951

Gatorbait, I think I'm having the same issue.

I've replaced bunches of stuff like vacuum lines, tps, wires, coil, 02 sensor, etc, etc., so I'm pretty sure it's none of the more common ailments.

Danno sent me a note suggesting that running rich could result in symptoms like this, but I haven't had time to do anything further about it. One thought was to try putting the 2.5 BAR FPR back in and see if it makes a difference. May be something for you to try?

Ron

eugene

Does the car " buck " as you go from full to part throttle (gradual) on deceleration ?

Overly rich mixture can cause that.

Also, a faulty O2 sensor can contribute.

Eugene.

Danno

I think the most likely culprit is fuel. Put the stock 2.5-bar FPR back in and the problem will go away. You have 9% more fuel with the 3-bar FPR. Combined with the already excessive mid-range fuel-mapping on the APE chips and you're well into the bucking & stumbling range.

"Does anybody else think it could be detonation, the car pulling back timing, and then accelerating for a little bit, then repeating the cycle???"
"Detonation is a possibility, but what's causing it?"

Would you believe that the APE Stg.2 chips dials in 59-degrees of ignition advance BTDC in the mid-range? Our chips actually have less ignition advance than the APE at every single data point.