# Engine cuts out at 5500rpm



## rconaway (Jun 3, 2016)

Bought a used 1989 240sx with a replaced motor. The car sat for a couple years. 

Although it starts normally and revs to redline for a few seconds, it quickly reverts to revving slowly and cuts out at 5500rpm, drops down to about 3500rpm and then repeats. We pulled codes and it shows that there are several things that caused the problems. 

We replaced injectors, head temp sensor, plugs, distributor, etc... Pretty much out of ideas at this point. Any other ideas would be helpful.


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## rogoman (Dec 16, 2004)

Post the codes here on the forum so that we can better help you. You may have a marginal fuel pump or the fuel filter is plugged up. Run a fuel pressure test.


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## rconaway (Jun 3, 2016)

I'll get them tomorrow but thank you for offering. We replaced the fuel filter and upgraded the fuel pump to a Walbro. We also added a fuel-pressure regulator and tested pressures from 25 to 45lbs. No difference.


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## rogoman (Dec 16, 2004)

rconaway said:


> We replaced the fuel filter and upgraded the fuel pump to a Walbro. We also added a fuel-pressure regulator and tested pressures from 25 to 45lbs. No difference.


When you tee-in a tempoarary fuel pressure gauge at the output side of the fuel filter. The readings at idle should be as follows:
- with vacuum hose connected to the fuel pressure regulator: 33 psi
- with vacuum hose disconnected from the fuel pressure regulator: 43 psi

If you're only getting 25 psi, that's not enough; it may give you driveability problems.

Here are some other things to check:

- The CAT may be plugged up. 
The easiest test for converter plugging is done with a vacuum gauge. Connect the gauge to a source of intake vacuum on the intake manifold, carburetor or throttle body. Note the reading at idle, then raise and hold engine speed at 2,500. The needle will drop when you first open the throttle, but should then rise and stabilize. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system.

- Check for a major vacuum leak in the intake system:
To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. A good place to connect to is the charcoal canister. There should be three hoses connecting the carbon canister. Two of the hoses go to a purge valve that's built into the canister. The second larger hose off the purge valve should be your vacuum line. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg.

If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
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Here are some vacuum gauge readings and their indications:
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Low & steady: Late ign timing/valve timing, low compression

Very low: Vacuum leak

High & steady: Early ignition timing

Gradual drop in reading from idle to higher RPMs: Excessive back pressure in exhaust system 

Intermittent fluctuation at idle: Ignition miss, sticking valve 

Needle fluctuates as engine speed increases: Ignition miss, blown head gasket, leaking valve or weak valve spring 

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