Wont rev past 3000rpm, went as far as replacing the motor with the same result.

[afiser]

My brothers 1990 240sx stock SOHC chewed up an engine harness in the fan and thats when all of this started happening. just after the harness was chewed we replaced the harness and it ran fine for a few months afterward then it started this bogging shit at ~3000rpm and it would not rev anymore above that (you also have to be extremely light on the gas or it will just bog aswell). Okay, so we went through the harness to find any discrepancies and it looked fine other than a few plugs we rewired to make a cleaner connection, it did the same thing. Replaced the MAFS with 3 different SOHC MAFS to no avail. Tried an entirely different harness and it did the same thing. The we looked at every sensor to see if there was a problem with the sensors, the ecu was no throwing any codes so this made finding a malfunctioning sensor a completely bitch, but the ECU would throw something if they were malfunctioning correct? Anyways. We went in for a DOHC swap got a motor, prep'ed it for install and installed it (obviously with a DOHC harness and MAFS etc.), and it does THE SAME FUCKING THING. after this we realized it was car related and not directly related to the engine (since we changed it entirely) we replaced the fuel pump (which looked in poor condition) thinking it was not supplying enough fuel or something wierd, it did the same thing, 3000rpm to boggggggggg. Anyways, we have run out of ideas, looking for anything that may bring new light to the problem and any suggestions, please help :sadwavey:

[!Zar!]

Problem with something maf related it sounds like.

[afiser]

Problem with something maf related it sounds like.

tried 3 different SOHC mafs when the SOHC was in the car, tried 2 different DOHC mafs after the swap. im not sure how likely it is that they all are broken

[tabasco122]

sounds like you need a new harness. if the maf is unplugged (or not giving the right signal to the ECU) then it will run in limp mode and restrict the rpms to 2600

[afiser]

sounds like you need a new harness. if the maf is unplugged (or not giving the right signal to the ECU) then it will run in limp mode and restrict the rpms to 2600

used 2 different harnesses (since the original, which was chewed up by the fan) while the engine had the SOHC, only tried one DOHC one tho, but the harness is in decent enough shape from looking at it. doubting its a harness or engine issue really, but will see if we can get another DOHC harness to eliminate it as a problem.

[Eramo323]

reset the ecu? disconnect the battery overnight

[MJenkins1101]

Could be the fuel pump is bad and as it tries to deliver more fuel it starts to bog down?

[afiser]

reset the ecu? disconnect the battery overnight

swaped to dual cam, therefore ecu was also swapped, also used multple ecu's when the single cam was in the car.

Could be the fuel pump is bad and as it tries to deliver more fuel it starts to bog down?

replaced the fuel filter to no avail (2nd to last sentence of first post)

one thing that did cross my mind was clogged fuel lines, how do you test fuel pressure?

and is it possible the ignition system is somehow messing up the spark?

[sidewaysonmy180]

Sounds like the fan isn't spinning correctly. If the fan stops spinning the ecu won't let the motor go past 3 grand. I know this by expirience. Haha

[Pro ViZIon]

check your fuel pressure and volume. definately sounds like the pump isnt delivering.

You need a gauge to do a fuel pressure test. You can do a fuel volume test with just a 2 liter bottle or something. Find out in the fsm how much fuel should run in what amount of time. Jumper the pump and see the results.

If you can get a fuel pressure gauge. At idle pressure should be approximately 34psi with vacuum hose connected to fpr and 43psi without the vacuum hose connected to fpr.

[afiser]

Sounds like the fan isn't spinning correctly. If the fan stops spinning the ecu won't let the motor go past 3 grand. I know this by expirience. Haha

not sure how that is possible since the fan is in no way connected tot he ecu at all

check your fuel pressure and volume. definately sounds like the pump isnt delivering.

You need a gauge to do a fuel pressure test. You can do a fuel volume test with just a 2 liter bottle or something. Find out in the fsm how much fuel should run in what amount of time. Jumper the pump and see the results.

If you can get a fuel pressure gauge. At idle pressure should be approximately 34psi with vacuum hose connected to fpr and 43psi without the vacuum hose connected to fpr.

thanks for this, will try this when we get a hold of a gauge

[afiser]

did a fuel pressure check and at idle (when it runs fine), the pressure was good, but once we gave it more gas the pressure went down? (i wasn't there but they did it without me). i think we narrowed it down to the fuel. any idea's on what may be causing it?

[Pro ViZIon]

Check your tps and see if its set where it needs to be. Other than that maybe you have clogged injectors? Your fpr could be malfunctioning. but i kind of doubt that.

[ki_JDM_ds]

For the record it's my car...

At idle the fuel was around 30psi, add throttle and the gauge drops to 0 and the engine bogs.

This problem was evident with my KAE. I swapped to KADE, therefor everything is "new". The KADE TPS hasn't been touched, the injectors and FPR never touched.

While looking at the FSM I notice a Fuel Pump Check Valve. Could this in some way be not functioning?

The fuel lines, trans, and muffler are the only items I reused from my KAE to KADE swap...

[DataXUnknown]

The only thing I could think of at the top of my head would be grounds. Make sure you have plenty of grounds connected in the engine bay. Also if the car is low, make sure your harness is not shredded where the fender is. A lot of the time tires chew through that harness and screw things up if you don't tuck it up all the way.

[ki_JDM_ds]

Problem solved! Replaced fuel filter with new 300ZX unit. Blew fuel line out with air. Revs great but is still running very rich.

Timing, MAF, and wires great. Plugs are black... Idle is a bit wobbly.

Upon inspection of my gas tank and old fuel filter I think it would be a good idea to replace my gas tank. Rust is everywhere and the fuel filter was clogged because of it.

[turbo-travis]

good to hear this, sucks you did all the troubleshooting though just to replace a part proboblly arround ten bucks.

[ki_JDM_ds]

good to hear this, sucks you did all the troubleshooting though just to replace a part proboblly arround ten bucks.

Yes, the reason it was overlooked was because replaced it back when all of this started and it didn't change how it ran...

So I spent like $500 on motor, new fluids, belts, exhaust, install stuff... The 300ZX furl filter was $12.99 lol

[ki_JDM_ds]

New problem...

It runs very rich!

So after replacing the fuel filter and cleaning the lines I drove it for a couple days. It took almost $10 a day to drive just 15-20 miles.

I recently fixed a leaky fuel delivery line, newer FPR, new fuel filter, cleaned fuel pump and blew out the lines with air.

The problem now is I pull the fuel pump fuse watch my fuel pressure gauge go to 0 PSI and the car will run and run a least for 15 mins. We tried clamping both to a return lines to try and kill it with no success. With the fuse pulled the car seems to idle better and it doesn't run rich but for obvious reasons you cant give it any throttle or it will die.

[clay9110]

So ur fuel pumps runs when theres no power going to it? wtf

[vas570sx]

i didnt read the whole thread, but i did read the OP. If you changed the MAFs, wiring harness and engine, the only other thing it could be is the ecu or fuel filter.

trust me.

nvm, i read the rest, i see you figured it out.

[daryl337]

That makes no sense whatsoever.

15 minutes of running with a 0 fuel pressure...
Where are you reading pressure from?
Buy a new Fuel pressure gauge. Haha. If you have 0 fuel pressure, your injectors wouldnt be able to supply fuel to the engine. Even if you were managing to scavenge fuel by some freak accident with a siphon effect, it would still need to bybass the pump, and pressurize in the fuel rail to provide a spray. Either your gauge is broken, or a gremlin has crawled into your engine bay and installed another fuel system that you do not know about.

[ki_JDM_ds]

So ur fuel pumps runs when theres no power going to it? wtf

Well I don't think the pump runs but the engine will.

That makes no sense whatsoever.

15 minutes of running with a 0 fuel pressure...
Where are you reading pressure from?
Buy a new Fuel pressure gauge. Haha. If you have 0 fuel pressure, your injectors wouldnt be able to supply fuel to the engine. Even if you were managing to scavenge fuel by some freak accident with a siphon effect, it would still need to bybass the pump, and pressurize in the fuel rail to provide a spray. Either your gauge is broken, or a gremlin has crawled into your engine bay and installed another fuel system that you do not know about.

I beginning to think my car is never going to get fixed. It's had some very unique issues and no body knows whats up.