Hesitation only when hot

[circaz1291]

Okay so to start off I have a 1990 300zx 2+2 NA with a manual transmission. It cranks over perfectly and runs great until it warms up, I can put light pressure on the peddle and it will accelerate and won't hesitate as much but as soon as I put anymore pressure or floor the peddle it starts to "hiccup" and it will jerk the vehicle a little bit each time it gets power but just bogs out and it gets worse as it heats up more. Now for everything I've changed already... All 6 fuel injectors, all rubber fuel lines, fuel filter, all vacuum lines, new z1 air filter with hard pipes, new knock sensor, new o2 sensors, new full exauhst, new timing belt kit with thermostat, new coolant temp sensor, new MAF, new battery, new CAS, new IACV, all connectors to the sensors, pretty much everything. Now what I haven't changed and what I might think is causing the problem is the TPS and connector, fuel damper, fuel pressure regulator, fuel pump, and maybe even the fly wheel and clutch. I do hear a clanking noise coming from around the bell housing and does go away when I put the clutch in while I me driving. Could the hesitation be coming from the clutch slipping? Sorry for the long post guys, pls help.

[evildky]

I would suspect the fuel pump. I've seen a few pumps that fail intermittently when warm, when you give ti throttle the vac drops and the fuel pressure raises this could be reaching the limit if your tired worn out pump.

[circaz1291]

Okay so I replaced the fuel pump with no avail. So now I'm thinking it's the clutch. Please help guys

[evildky]

A clutch does not cause hesitation, it causes slippage. the engine will continue to run fine but under load, say 5th gear uphill low RPM's the engine speed will raise but the car speed will not.

[circaz1291]

Then what would cause hesitation only after the car warms up and at low rpms? Its almost a jerking feeling and I have to put little pressure on the peddle for it to actually go anywhere and more pressure will cause it to hesitate.

[nissanfreak12]

Bad coolant temp sensor(even though its new, still could be bad)
Ignition timing bad
Bad or out of adjustment TPS
Vacuum leak
PLug incorrectly installed on TPS(basically the black plug installed backwords)
Bad fuel pressure/fuel pressure reg

Here is what to do:

Test fuel pressure
Check ECU for codes, don't rely on a CEL
Double check all plugs to make sure they are installed correctly, some can be switched but shouldn't be

[dzap]

Having the same exact same issue here. The behavior is I can crank the car, starts properly, idles properly, and can start driving it immediately. The gas/acceleration response is awesome when cold, it's very responsive even to light inputs to the gas pedal.

Once the vehicle warms up, if I park it and turn it off for a few minutes (going into a store for example), come back out and start it, the acceleration sucks from standstill. If I jam the gas pedal it hesitates, chokes a bit, and then finally responds and is fairly responsive from then on once at speed.

If I let the car sit for the next 8 or 9 hours and cools down (while at work), I can turn on the car and the car is once again super responsive....until it warms up.

Here's the relevant info.

- 1993 300zx NA, automatic.
- All 6 fuel injectors were replaced in the last 600 miles.
- Wiring harness replaced (will have to review the paperwork to clarify).
- Oil, oil filter, and both air filters changed last week.
- New fuel filter changed last week as well.
- Fuel pressure was checked immediately after the fuel filter was replaced.
- Cleaned MAF with MAF spray.
- Coolant was checked and topped off the same day the oil was changed.

Thinking about checking/replacing the following next:

- Battery
- O2 sensors
- Spark plugs + ignition coil packs

[dzap]

Here is a page to read. It has a lot of useful information - http://www.zhome.com/ZCMnL/PICS/hesitat ... ation.html