Car cuts off at 4500 rpm

[Vegascorbin]

1990 240sx KA24E S13Car has been converted to a full time Dirt track race car. Full stock engine and intake, header and collecter only on exaust ( no cat or muffler)

Car runs ok untill we hit aprox 4500 RPM in 3rd and then it cuts out as if the fuel is cut off. Let the RPMs fall to about 2500 - 2000 and it starts running again. Runs fine until we hit about 4500 and then it does the same thing. It also cuts out in first, second and forth gears. We do not have the room to get it into 5th.

When it cuts out we just keep our foot where it was. We do not shift or anything. RPMs drop to 2500 or so and it take off like nothing ever happened. We do not have to touch the starter or anything.

When we first got the car it would cut out at around 3000 both in netural and in gear. Now we can hit rev limiter at idle.

What we have done so far:

Replaced stock fuel tank and pump with racing fuel cell (required by track)and Walbro 255 in line fuel pump. New Bosh plug wiresNew spark plugsNew cap and rotorReplaced Manual Trans due to reverse gear problem (not thought to be related) new clutch,flywheel, pressure plate, throwout bearing (not wired in).New Fuel Filter (stock size don't remember brand)Replaced aftermarket Ford Electric radiator fan with stock clutch fanReplaced TPS

At this point, we know now, it was only running on 2 and some times 3 cylinders and no noticable improvemnet in the problem.

Replaced Timing chain (gears, front seal, tensioner, guides) due to standard 240 engine rattle.

At this time we swapped the Distributor with one from a parts car we have.

By now the cut out point had rased to 3300 to 3500.We swapped out all 4 injectors with used ones from E-Bay. now it ran on 3 cylinders.

The engine then developed a bad knock at just about any RPM. This ended with catasrophic engine failure (#3 rod through the side of the block)

Did a full rebuild on a replacement block. Bare block on out. All parts wer either replaced or checked by a machine shop. Stock rebuild.

Removed Swirl valves and acuator. Removed all polution control devices (EGR etc) and vacum lines. Only remaining vacum lines are one from manifold to stock FPR and power brake boster. All others are capped.

New engine ran fine but would still cut out but now it was about 3500 to 3700 RPM. Swapped coil and Power Transistor with parts car.

While checking we found that #4 injector was not firing. Ulimatly found this problem to be a faulty ECU. Now the car would run on all 4 cylinders.While trouble shooting the injector problem we found the main engien harness to be flakey (move wires and rpm changes). Replaced harness with one from Parts car.

After changeing harness and ECU it now starts better and the cut out point is up to 4000 RPM.

We changed the distributor with one from the junkyard and then the cut out point went to 4500 RPM

We reinstalled the ground from the back of the head to the firewall (it was missing) along with adding a ground from the drivers side motor mount (block side) to the fender well near the Coil.

At various times we have tried 3 differant MAFs either from our parts car or Junkyard, and 3 differant ECU's. changed both sensors at the front of the intake with ones from the junkyard. A new radiator and thermostat were installed at the time of the engine rebuild. We have replaced the O2 sensior but don't remember exatly when.

My research leads me to belive that the ECU runs on pre-programed settings untill the load reaches pre-set points. Then it activly looks at the sensiors to set fuel timing etc. It seems as if when it switches over to "active" mode it cuts the fuel.

ECU shows no errors (55 code) but we do have a total power cut switch (required by the track) so every time we power up the car the ECU is getting a "Fresh" start.

We run in a "Pure Stock" class so things like add turbo or swap in a DOHC are not permitted.

Thanks for reading this far. Please help. We are out of ideas.

[Vegascorbin]

Bump

Not even a sugestion on where to look for the problem?

[silviaS13milknhoney]

I would try adding at least one more ground from the block to either the frame or suitable location. That and make sure your ecu ground is good. Most likely not a fuel issue or a spark issue. Hope this helps a little

[Vegascorbin]

Thanks for the response.

By not a fuel issue I assume you are refering to not being a problem with the pump FPR or Injectors.

[silviaS13milknhoney]

Yeah, have you replaced your FPR? Its most likely not that, i just had to replace mine due to it not starting when hot. I have the dohc but mine would still rev to the 7. Just seems like you have a ground issue, either than or an ECU problem. I don't know how many modules these cars have. Maybe get a multimeter and start checking resistance on some grounds. Anything more than a 1 ohms is usually a bad sign ( depending on the gauge and length of wire of course) Let me know if this helps at all

[Vegascorbin]

Thanks for the input.

Question: We are running just a Header and collecter for the exaust. everything from the front of the Cat Converter back was taken out before we got the car. Could this be affecting the ground for the O2 sensior? Or does the car require the backpressure of a cat etc to run properly? Being an off road car the lack of emissions is not an issue.

Probably wont get to speen much time on the car till the weekend. I will let everyboady know what we find.

[silviaS13milknhoney]

i dont think that would effect a cut out issue. The o2 senser only relays how many hydrocarbons arent being burnt to the Ecu. Meaning if the engine is running too rich or too lean. At WOT the o2 is ignored and the ECU will enrich the fuel/air ratio. However this i believe would not cause a cut out issue. So you are not running a o2 sensor at all?

[silviaS13milknhoney]

i am currently in school at Gateway Tech here in ky, fantastic school. But when i return to class on monday i will ask my instructor what he thinks. He worked for nissan for 5 years solely doing engine performance. If he doesn't know that i dont know anyone that would. I'll let you know.

[Vegascorbin]

Yes we have an O2 installed but we are not sure if it is reporting back to the ECU or not. That is #1 on our list to check either tonight or tomarrow. I did look up the US patent Nissan got for how the fuel delivery system works. The O2 sensor is factored in reguardless of throtle position. How big a factor it is i don't know.

As best I can tell, the ECU considers Throtle, RPM, MAF, O2, Water Temp IN THAT ORDER of importance when calculating spark and fuel. Under heavy acceleration (determined by throtle and RPM) MAF and O2 are the big factors as the ECU does not use any preassigned values.

That said, I think my problem is either the MAF or O2 signal is not reaching the ECU or the value being received is too far off due to either bad voltage or grounding. We have our work cut out for us this weekend.

Thanks for yoru input and any further comments and/or ideas are appreciated.