Techno Duck

This is a tough one to explain because it happens at strange times. Basically i noticed this during a long 150 mile or so trip to the track a couple months back and it pops up every once in a while.

Here is the problem, while accelerating the car feels extremely anemic, almost as if its close to a misfire.. however it never actually does misfire and the AFR's remain in the correct range despite this strange feeling. It typically happens under any acceleration, be it light, or rolling on the boost right as i come out of vacuum into boost. As soon as it goes full boost the car pulls with no problem...it only seems to be in that transition period of part boost does it happen. Its sort of a surge.. like you feel the car lag and not have the correct output for your throttle input, then it sorta catches up. The best way i can put it is you give the car a certain amount of throttle and you get maybe 1/4 of the power output you are expecting.

What has me stumped is it happens at very strange times. I first noticed it after a long stretch on the expressway heading to the track.. about 25-30 miles at constant speed. I would downshift to 4th to make a pass and noticed it. Same story when i came off and was driving through some traffic lights. I thought it was initially bad gas. But when i arrived at the track and let the car sit about 20 minutes and restarted.. i had no problems at all. Car ran fine on track also and problem did not come back until the ride home (same circumstances). It will also have the same problem on completley cold starts in the morning at random.. same lag / surge the entire way to work even after completley warming up (about 20 mile drive)...shut car off and restart about 3-4 hours later when i leave and car runs perfect? What gets me is that its almost completley random when it happens in the morning.

Any ideas what this could be?

I have replaced the cap, rotor and spark plugs (NGK BPR7ES with .028 gap) all within the last 1k miles to try and cure problem. The wires are Magnecore and though are a little beat, dont have too much mileage on them (cant be more than 30k i dont think). I have checked for vacuum leaks and everything seems to be fine. Warm idle vacuum is 19-20''hg. The last thing i could think is maybe a weak coil or maybe i should replace the Magnecore wires?

I should also mention car is equipped with Vitesse MAF, 3bar FPR, Tial 38mm in dual port mode with LR black **** controller. I never noticed the problem with the K26/6..car currently has 60-1 Hifi with #8 hotside. As far as i know the intake system has no leaks. Maybe i can convince my friend to let me smoke test it tonight to make sure.

fast951

If the AFR is fine (no weird readings), that leaves the ignition side (or electrical issues).
Go back and note when the problem started first, does it coincide with any other change?
Next, figure out how to reproduce the problem at will, list the events required. As long as you can reproduce it at will, it will be much easier to figure out.

Techno Duck

John, i just drove the car and everything is currently running fine. I did just notice that i was mistaken with where the AFR's should be in that part boost / transition phase where i was having issues.

The car is running fine right now and i noticed my AFR's go very quickly from low 12s to 11.7-ish as i roll on the boost. With the intermittent lag i was feeling the AFR's are dithering in the low / mid 12's. I guess its running lean? This is what it seems like right now atleast, i need to drive the car a little more and see how it reacts when its 'lagging' to know for sure.

Tom M'Guinn

This reminds me of an old Steve Martin joke where he promised to teach you "how to become a millionaire and never pay taxes. First, get a million dollars, then...."

If possible, it might help to do some data logging to see if anything shows itself.

ehall

Fuel filter?

Jeff N.

Interesting. I have had surging issues on another motronic car where the FPR was sticking. Here's what was happening.

The FPR would stick, lowering the FP by about 10 PSI. This would cause the car to run lean. At mid throttle, the car ran closed loop so the ECU would see a lean condition and then richen the mixture to move it towards stoich. This created the surge as the AFR dithered. Car would not surge at WOT as the ECU went open loop and stopped any corrections.

Not sure how the 951 motronic runs at mid throttle but it seems that your problem is similar. If it worked anything like my other car, you'll see a pretty big dither at mid thottle in the AFR's at about 1 second intervals.

fast951

If the problem is intermittent, keep a log of when it happens. Hopefully there is a pattern, which will help you identify the cause.


Mid 12s AFR is not lean. However, if you were getting 11.7 under the same conditions then something is causing the engine to run leaner. (fuel pump, filter, bad ground,..)

The more data you gather the easier it is to diagnose.

Techno Duck

Thanks for the input as always John. I just checked my records and the fuel filter only has less than 25k miles on it so probably not a bad idea to change.. it was also last changed in 2002 so maybe a really good idea to change it.. . Magnecore wires were put on at the same time. I will hopefully be able to report back if it still happens with some better info to help diagnosis.

I have been holding off on buying the data logging box from Innovative.... so much to buy but so little time and money.