I’m trying to diagnose an intermittent loss of engine power while driving. It happened for the first time today and continued to happen intermittently for over an hour. It started after driving 3+hrs, in the Sierras near Yosemite (between 7000-8500 ft) with outside temperatures in the mid 80’s and the car running flawlessly at around 210F. While on throttle (light or heavy) the engine would just quit. If I put the clutch in it would stop completely as if the ignition were switched off. After 2-10seconds the engine would kick back on at whatever throttle level was set, as if nothing was wrong, and continue on for between 30sec to several minutes before it would happen again.

The issue finally seemed to stop after we coasted down a grade for about 5 miles (engine braking the whole way). Over the distance of the grade the coolant temperature came back to a more normal 185f.

The issue would happen in any gear, at any speed and even if the clutch was depressed. It died at a stop once also, but started right up with the key. No codes were thrown for the OBD2 system and no warning lights flickered or came on.

This is a base 996 C2 with no PSM.

Any help is appreciated!

 

Maybe Bad ignition switch.

 

I ran the car this morning, hard, and it performed flawlessly over the 40min drive with RPM’s kept between 4000-6000. The temperature of the coolant never got above 190, and the air temperature is in the mid 70’s. ...still around 8000’ of elevation.

 

Fuel, spark and air

At elevation air could be a contributor but with our modern cars not likely. Low fuel and a combination of high elevation?

Fuel pump struggling on twisty road if fuel low could be the cause.

 

Yeah, your fuel pump may be fizzling out. This theory is further reinforced by the fact that there is no check engine light or codes.

 

Thanks for the fuel pump suggestion. It was slightly above half a tank when the issue occurred, but I’ve had it above 3/4 full since then.

I wouldn’t think vapor lock would be an issue with this type of EFI, but have others reported this type of fuel cut?

It’s a sudden off-on with no hesitation or reduction in power.

 

You're welcome, matz. Before my fuel pump gave the ghost, my car intermittently stalled. Turning the ignition key would usually fire up the engine right away. Those symptoms occurred for a couple days before I was not able to start the car at all. Fuel pumps are easy to remove and test when there is no ignition. The intermittent symptoms you are experiencing make diagnosis more difficult because the car still runs most of the time.

 

With the loss of power at speed for an extended amount of time, 2-10 seconds, and NO CEL/CODES,you can eliminate a few things. You can eliminate the CPS sensor or any sensor that is monitored while running. You may also can elliminate the fuel system, as running lean/no fuel for 2-10 seconds SHOULD produce a lean running/fuel trim code.(although this should be verified by watching it with an ODB2 device while it is happening)

The LOGICAL explanation would be a loss of power/potential of the ignition switch/DME relay/wiring to the DME.. Interruption of this circuit would cause the same as just turning off the switch, and cause NO CODES.

 

First thing I would check is your intake setup. Make sure the hose clamps are tight and there aren't any leaks.

I had the same issue with my 240SX. The intake tube was not on tight.

 

Yep, this is also possible, huge amounts of unmetered air entering between the MAF and TB would cause a power loss.

 

...improbable to happen only after extended driving I say and it would throw codes, wouldn't it?

The opposite to unmetered air as in too little or none could be causing power loss when the rubber intake hose collapses at higher engine compartment temperatures and low engine load/high vacuum conditions. But I have never heard of that related to 996.

I am with you on your earlier post though: It's electrical.
@matz73z Anything that causes a sudden loss of supply voltage to the DME like a faulty relay, ignition switch, loose battery cable or ground wire.

 

Yep it SHOULD throw a code if the specified MAF( calculated from RPM and throttle angle) is too far away from the actual MAF value,but in reality we have seen cars run VERY poorly from a faulty MAF value and not set a code, although not as bad as described by the OP as in complety shutting off. I agree it's improbable for extended driving to exacerbate the symptoms, but not impossible.

The factory intake hose is thick and ribbed, I don't see it collapsing even with a very restricted/stopped up air filter and high temperature. The aftermarket intake hoses are thinner and more flexible, I could see them collapsing under high temperature for an extended amount of time with also having a restricted/stopped up air filter.

 

When troubleshooting a car, I always check the common/free/easiest possible causes first. It has served me well.

I bought two cars where the previous owners thought the engines were blown. Fixed one for $30. The other with an oil change and cranking it for 30 seconds.

I had another car that kept blowing the rear tail light fuse. I changed all of the rear tail light bulbs and it fixed the problem.

I just fixed a car that wouldn't stay running after starting. No CEL. Lots of people told ne it couldnt be the IACV because of no codes. Was the common stuck Idle Air Control Valve. Fixed for free.

 

Mice