Slow...then fast

[Frozen240]

Here's my setup-jwt ecu, 72lbers, z32 maf.

My car has run good three times now. All three times the engine has been cold and the ambient temp was about ~40F. It was making awsome power for 7psi. Spinngin the tires all the way through first and well into second gear. Then after 15min of fun it's slow again, as fast a switch. It seemed to loose about 30rwhp, by a seat of the pants guess.

I'm really at a loss to figure this out, so I thought I would post it in case someone has had a similar experience.

I have checked out the maf, tps, knock sensor, engine coolant temp sensor, and all are good. Fuel pressure is good (walbro pump) and plugs, rotor, cap, wires, fuel filter are all new. There are no vacuum leaks and the car idles suprisingly well. One thing that it does not do is start when the engine is hot, I usually have to remove the fuel pump fuse to start it when the car is warm, if not it will just flood out.

The base timing is set to 20deg. I do not have any way to check the timing under WOT. I'm not sure if the JWT ecu is pulling too much timing, but why would it do this 98% of the time and then run aggressively when the engine is cold for 15min?

I have a wideband and the AFR's are all good under boost, 11.5-12. They never changed.

I think I've said everything relevant to the situation, hoping that you guys could give me some hints cause I don't get it.

[Jookmasta]

i would like to know more info on this loss of power........ i know that with my car, after a good couple of runs, it feels like it lost some power also but i attribute it to the oil heating up or the transmission heating up or something is heating up lol. as for your not being able to start up the car with the engine hot, does the engine crank over? any weird noises? more info on that too......

[nismo1003]

check all the piping, maybe there's a leak.

[NateDogg]

As coolant temps increase, timing is reduced to prevent detonation. What is your cooling system like?

That starting problem sounds like when I had a leaky injector. Pull your plugs and see if there's one or two injectors that are dripping when the fuel lines are pressurized.

[DammitBobby]

There is a temperature sensor for the ecu. You might want to check and make sure it has a good connection or the sensor is working.

[DRIFTEADOR]

there is a coolant vs fuel table too. if it was related to coolant temperature your afr would also change so i doubt its that. I had an intermittent(sp?) problem similar yours a while back and it turned out to be the igniter. one day on the highway it started backfiring and stopped working completely. left me stranded 200 miles away from home try borrowing a friends and see if it fixes it.

[get_up_mark]

iv heard people say their car has more power before it warms up also. im curious as to the reason. but if your spinning your tires and driving hard for 15 minutes i think that ur car is way past warm up at this point. ur car could be over heating

[deezlins]

running your car hard for a while heat up the turbo and engine bay alot and your intake air temp will rise, and therefore you will have less power, but if your car doesnt start when its hot i expect theres something else afoot btw, what kind of intercooler do you have?

[Frozen240]

Well, I replaced the engine coolant temp sensor. Kind of a long shot to begin with, nothing changed. The onloy other thing that I have been able to come up with is the ignition coil doesn't have the proper resistance. The service manual for the 92 240SX says that the primary resistance should be ~0.7 ohms. Mine reads 1.4 ohms. Although it is only 0.7 ohms greater it is still double the factory spec. Is this too high? I'm just not sure what (~) would be.

Should I replace the coil with an MSD unit?

The other thing that I'm thinking is that it is the JWT ecu. There might be something amiss there, or it's just tuned that conservative. The car really doesn't feel much faster than stock. Pretty pathetic for 7psi. I think I have to get this on a dyno and check the total timing before I can come to any conclusions.

[Frozen240]

Well, it's not temp related, and it's not a vacuum leak. I regapped the plugs to .035", didn't make much difference.

I think that I did fix the problem. On the vacuum gallery on the underside of the throttlebody there are four hardlines. I had them all capped off as I had removed alll the emision controls from the car. Two of these are vacuum lines, one is a purge for the carbon canister and one is a atmospheric vent (I believe) for the AIV system. I'm not exactly sure which one is which, except the vac lines. I unplugged the second closest line to the engine, I think this is the atmospheric vent for the AIV system. After I did this.....magic! The car finally runs as it should! Lots of power and quick spool up. Night and day difference, I'm so happy that it was something so simple.

I thought maybe I had a bum ecu tune and was looking at different standalone options. I cring at the thought of spending another couple grand on the car right now.

The car does run rich, wideband even reads RICH under full boost. Cruising AFR's are good at ~14.7 and it idles great.

[jab11185]

i had a problem in my 200sx where the alternator wires were corroded to the point where it would slow charging and i expericanced a loss in power similar to what you described...great a cool temps (less resistance) and as soon as the motor heated up....no power, it continued to the point where the car stalled and had to be cooled down and the battery recharged some

probably not the problem but it's worth throwing out there in the odd chance that it is....

[Jookmasta]

glad to hear that the problem is fixed. it seems that just plugging up any and all vac lines can do more damage than anticipated. i plan on tinkering with some of these tomorrow myself.

now get to the dyno!