Power Loss in 5th Gear
 

While cruising in 5th gear at approximately 70mph I noticed the car beginning to slow. I began to give the engine more gas and the RPM and speed of the car continued a slow decline. I shifted to 4th and then 3rd while attempting to gain RPM and maintain speed. Even with the car in neutral, I was unable to rev the engine or create any change from idle. When I pulled the car off the road it idled in neutral and there were no unnatural noises or knocks. I then turned the car off and, after checking for fluid leaks and unnatural smells, I started the car. I was able to throttle up and control RPMs just as I normally would. The car performed beautifully for another 45 minutes before the same problem arose. The car then ran 2.5 hours without any problem in 5th gear on the highway under normal driving conditions. No warning lights ever illuminated and so far no one has been able to answer my question as to what went wrong. I surmised a computer/electrical issue since the problem was removed by turning the car off and restarting. The throttle linkage is in fine working order and there are no other obvious physical signs of wear or damage to the engine. Has anyone else had a similar problem or can anyone point me in the direction of the problem/solution?

Thanks!

What is your cars model and year?

Sorry, new to all this and got caught up writing about the problem. I have a 99 Boxster

Consider the fuel filter - when was it last changed?

You may want too check your MAF unit. That happened to me when I first purchased my Boxster and the problem turned out to be the sensor.. :matchup:

I thought about the MAF but the question arose: Would the problem be confined to 5th gear and be so intermittent? I know I have a throttle cable and not e-gas which rules out a throttle body and I am hesitant to have my mechanic run a diagnostic in fear of "Trouble not identified" since the CEL never came on. I can replace the MAF but would like to avoid the cost if unnecessary. Is there anyway to check the MAF wihtout running a diagnostic? I am a Mechanical Engineer but have limited experience working on vehicles.

Thanks for the help,

Luke

It doesn't sound to me like it's related to 5th gear directly, but related to time and potentially temperature. I'd tend to suspect the fuel pump first. It could be that the air temperature sensor (part of the MAF) is malfunctioning as the car warms, but that wouldn't be my first attempt. The dealer may be able to measure current draw from the fuel pump when it's warm to see if there's a problem. What's the mileage on the car?

I'm an ME too, go Boilers!

The mileage is 84k with service at all recommended intervals. I knew the previous owners and they took excellent care of the vehicle. The temp could be a key, the only time the car faulted during daytime running on the highway (about 80F outside temp). I drove it at night for a total of 3-4 hrs this weekend and nothing out of the ordinary happened. What I keep coming back to is the lack of a CEL. Would the MAF or fuel pump trigger a CEL with a fault like that? Are there any other conditions that would cause the engine to default to idle and remove throttle control?

Go Tigers!!

I had the dealership run a diagnostic on the car and they found nothing wrong. Are there any further steps that could possibly lead to a proper identification of the problem or do I just wait for to to happen/not happen again?

Thanks

Could this be a faulty Throttle position sensor? I would think a CEL would show if that was the case, but I have spoken to several mechanic friends and they have all mentioned the TPS.

In cars that use a MAF to meter fuel (or a MAP for that matter), the TPS is used to adjust mixture for transient conditions (like almost no throttle to quickly full throttle) and to tell the ECU when the throttle is closed. I'm not sure that in a fairly steady state operation, like cruising, that the TPS has any influence. It could still be the problem, but I doubt it.

The TPS is very easy to check, it's just a potentiometer, all you need to do is know what the resistance should be at certain throttle positions and you can connect a multimeter to it to see if it's reading correctly.