Flat spot and detonation

[Clay Perrine]

All,
I have a highly modified 964 motor in a 914, so I thought this would be the place to ask about my running issue.

The engine has been modified with LN Engineering Cylinders, JE Pistons, 993 RS cams, 993 Injectors, and a Steve Wong custom chip. It runs great and pulls really strong. But if I am just cruising along under 3K and try to accelerate, it falls on it's face. Huge flat spot. It even happens when I hold the idle up and try to pull away from a stop sign. Plus, if I really hit it hard, it briefly detonates.

I am thinking it is going lean for some reason, but the DME doesn't generate any codes, and the fuel pressure and is correct.

Anyone have a suggestion on where to look?

Thanks,

[ian c2]

Knoc sensors kicking in due to pinking ?

[tailwind22]

Was the chip custom programmed by Wong based on all of your modified specs? Sometimes hard to map all of the spots in the program without dyno time.

[Clay Perrine]

Originally Posted by tailwind22

Was the chip custom programmed by Wong based on all of your modified specs? Sometimes hard to map all of the spots in the program without dyno time.

I gave him the engine specs, and he wrote the chip. I am thinking it may need some tweaking.

Maybe I should consider an MS3 Pro board in an old 964 ECU case???

[blue911rsr]

Does it still have the distributor belt ? Maybe it is broken seen lots of running issues when
That happens

[kaaayelll]

Have you chased this down any further?

[Clay Perrine]

Originally Posted by blue911rsr

Does it still have the distributor belt ? Maybe it is broken seen lots of running issues when
That happens

I spent some of this afternoon checking out my car. The belt in the distributor is good, and I checked the wiring to the two knock sensors. They are all intact. I also fired it up and put a timing light on the coil wires. Both are firing.


I have two ideas at this time. One is that I have a failed knock sensor. The second is an intermittent in one of the coils/igniters. Meaning when it gets hot, it fails. I am currently considering the 993 coil conversion and new igniters.

Does anyone know how to test the knock sensors?

Clay

[mikedsilva]

Subscribed... I'd also like to know how to test a knock sensor to see if it is working. I'm guessing connecting a multimeter and measuring resistance?

Or connecting your multimeter and setting to AC and tapping the sensor to see if it induces a voltage?

[Kraftday]

Clay,

It sounds like ignition wires and coils could be tired. The spark plug wires are pricey because of all the holders and sleeves so usually they get reused. The 993 coil update to replace the cylindrical coils is a great update. But also want to check and adjust your crank pulse sensor gap.. try increasing slightly or update to the newer version if old. The plugs on the crank sensor, knock sensor, etc are updated as the sensors are renewed. Also there is a tecnical bulletin about intermittent connection at the air flow meter plug.

The air flow meter itself should also be tested for consistent resistance. The sweeper arm can wear down a track on the potentiometer pad giving dead spots.

To test the air meter, you manually push the arm while ohm checking. If hickups, you can crack the cover, polistion the sweeper arm on a fresh track.

[Clay Perrine]

I am finally getting back to this after dealing with oil cooler issues.

The plug wires were new when I built the motor, and I have had this same issue since installing it. I did the 993 coil conversion after losing one coil.

The crank sensor is the 993 version and was installed new when I built the motor. It was the main reason I had issues getting the engine to start at first. I crossed the signal and shield wires when I installed the 993 plug in the harness.

I am familiar with the AFM issues. I have spent lots of years keeping a 914 with L-Jet alive, and they have the same AFM. I will check for issues with the wiper contacts in it and see if that is the issue.

Yesterday, I put an LM1 on the engine and drove it around. It only happens when warm, and only below about 3k RPM. But when it happens, it is like carburetors with a dead accelerator pump. The data log shows the car going extremely lean, and it falls on it's face.

I am going to try putting the stock chip back in the ECU and see if that makes a difference.

I have also been considering the idea of getting an Megasquirt MS3Pro board and putting it in a stock 964 ECU case so I don't have to build a new wiring harness. I could eliminate the AFM completely.

Thanks for the help and ideas.

Clay

[Clay Perrine]

For all of you that helped me, thank you. I got it resolved. Turns out the Air flow meter had a bad spot on the board. I would not have thought to check that until I got the answers here.

Clay