During warmup and imediately after the engine seems to pull fine. After about 10 - 20 minutes on the highway the car feels much more sluggish, and once i am off the highway it takes considerably more throttle to get around town.

I have changed my in tank fuel filter, but that did not fix the problem.

Anyone have any ideas?

Here's the list of things I'd guess matter:

+Engine Does Not Throw Codes or Have CEL Light
+Temp Stays right under the middle of the gauge
+Oil pressure normal
+High Pressure fuel filter has 8k on it and in tank sock is new
+K&N Air Filter has 8k on it
+10w30 Oil has 1.5k on it

 

i experienced the same problem yesterday, i wonder if its not just all the heat under the hood because i turned the car off after the highway and 15 mins later it ran like normal. then when i got off the highway again i noticed the same thing, but not as much this time. fix: CAI!

 

id think that if i didnt already have a CAI... it could be another engine part or sensor becoming unstable in the heat...

 

How did your tank look when you changed the pump filter?

Since the main filters are so cheap (aftermarket- Fram G6566), you could try that, if your tank had some crap in it, it conceivably might have clogged.

Sounds exactly like the symptoms of a clooged filter (one of 'em, anyway)...And after you let it idle for a bit while she's "acting up", you can rev fine in neutral, right?

 

Yea, check the fuel filter again.

Also, when was the last time you cleaned/changed your plugs & wires? I know on mine, just taking the plugs out & cleaning them can make a big difference. If the wires are old, they may be starting to crap out under heat. If you have a timing light, put it on a wire & check for misfires when it's being sluggish.

-=Russ=-

 

I have the Same exact Problem! but it was not over heating. however the Heat from the radiator was so hot behind the fan i had to take my hand out. i put it in neutral and it even revs slow.

 

in neutral it revs fine... the plugs and wires are new.

do you think it could really be the main fuel filter even though it only has 8k on it?

 

A longshot - Is maybe the clutch slipping?

 

That's why I asked you about your tank condition...Hell, you can clog a filter in 100 miles if you have enough sediments suspended in your fuel...

And if it revs fine in neutral, that's even more evidence of a filter clog...Or something blocking fuel flow, anyway...

Do you smell your clutch? Just in case SureShot nailed it...

 

nah, clutch is new, and id notice the rpm's jumping up

the tank looked ok when had the pump out... looked surprisingly clean

i just reset the ecu and took it for a spin... still felt the same, power in the begining and then falling off the longer i drive... didnt feel like it was as bad when i was coming back from the highway though as usual, but it still felt weaker

also, the car seems signifincantly louder on the trip back from the highway than going to the highway... im not sure what this might mean

 

Please explain? I can't understand reving fine in neutral to prove that there is a clogged fuel filter.

I would imagine their is heat issue. Some overheating issues with my car have made me notice much less power. I would check the cooling system, because heat is the only thing that will consistently rise after driving.

 

the temp gauge stays right under the middle... im not sure how accurate that is, but it doesnt seem to have an overheating problem... im not sure how else i would check

I just rechecked my compression, and thats fine, and i buzzed out the relay and resistor for the fuel pump and those were fine.

Anyone have any ideas what I should check next?

 

kerbump. This thread suddenly interests me again. After taking my car out for some high power work (engine's broken in, I'm in the process of learning how it handles at it's limits), there was noticeably less power after running it hard.

There was a thread somewhere, and I can't find it, about rotary aviation seals & compression, basically discussing the fact that as the seals heat up, they expand and don't seal properly. I can't find it for some reason, but I'm thinking that might be related - I have RA seals in my rebuild.

-=Russ=-

 

Hmmm, interesting...The only way to prove that would be if you had some really weird wear marks on the side housings where the ends of the seal might be riding. They would expand slightly faster that the aluminum housings, but you gotta remember, they run these seals in aircraft applications, where the rpm's are always "up there"...

New concept to think about, though

 

Ok, edit is silly.

I can also hear/feel a significant difference in pedal to noise ratio. When it's colder, the engine has "bite" to it - there's a noticeable change in power & noise with a small poke at the gas pedal. As it heats up, there's much less of that. The pedal feels lighter, and there's not the same growl with a small throttle movement.

-=Russ=-

 

The seals have the angle where the parts join, so they wouldn't rub on the housing - they'd just slide a bit, and the corner piece would be farther away from the housing, so lower compression.

I'm also planning to check on my spark plugs (tomorrow, once the engine has cooled). They could be having issues with the heat.

-=Russ=-

 

You're sucking in hotter, "lighter" air when she's hot (even with what we call a cold air intake), therefore the AFM and intake temp sensors are telling the ECU not to schedule as much fuel, because there is not as much of an air charge (less "mass") when she's hot...Have you ever felt the temp of the round black intake tube leading to the TB elbow after running her a while?

 

Nah, I don't buy the RA seals theory, mainly due to the fact they "float" on the springs, therefore any thermal expansion would be negated, and as far as lengthwise expansion, like you hinted at, there's plenty of room for a difference in expansion coefficients between the dissimilar metals...I think it's just "hot air", I mean, everyone can notice a power difference between a cold morning run and a hot afternoon jaunt...

 

Yea, I've noticed that. I wasn't sure how much of a difference it would actually make... still learning all this car stuff.

Has anyone put an intake air temperature sensor on a NA to find out what sort of air is actually making it into the engine? I imagine after rocketing through the intake runners & such the air gets damn hot. Would this be enough to cause a noticeable lack of power? Also, would water injection be a possible solution to this? I'm planning to do autocross & some track work in the next year, and would much prefer not losing power as it heated up.

-=Russ=-

 

I thought about it, the problem with an intake sensor, is you'd have to keep it out of the "moving air", or the readings will be skewed...Kinda like our "official" temperature thermometers at airports, they're always in a white box, out of the sun, with 3 sides of the box closed in to negate any wind effects...

 

How do the turbo guys do it when they're testing intercoolers & such?

As long as the conditions are the same, it shouldn't matter - even if the readings are slightly skewed, it would still be able to be compared. "Hey, I'm sucking in 90 degree air when the engine's just warmed up, and 200 degree air after a hard run." Also, unless there was moisture on the sensor, a moving air stream shouldn't affect the temperature reading.

I'm perfectly willing to instrument my car up & generally make it geeky, I was already planning to find a way to display fuel injector duty cycle on bar graphs. I suppose that would be a good indication of how much fuel was going in at various temperatures as well.

Water injection on turbos is used to help cool the intake charge, among other things. If the problem is indeed just hot intake air, water injection should be able to help reduce the intake temperatures on a NA as well.

-=Russ=-

 

Yeh, I've noticed several water injection threads lately- do a quick search (hope that works better with this "latest and greatest" format we've got here, lol)...

In the FSM there is a table that compares actual intake temps (in the dynamic chamber) to the voltage output of the intake air temp sensor, so in theory you could actually hook up a meter to the input at the ECU, and track the voltages during changing conditions, then using a little algebra get a reading in "english" from the table. Think it's in section 4A for us N/A guys...

 

I found the table, but it appears the sensor is in the airflow meter. This isn't the best place to find the intake temperatures IMHO, since there's still the long air tube and the blazing hot intake runners. A better place would either be in the rubber right before the throttle body, or even better, at the end of the intake system, right before it went into the block. Drill & tap the lower intake manifold or such. Probably a bit overkill, but certainly the best location for finding out the temperature of the charge going into the engine.

-=Russ=-

 

You've also got one on the left side of the dynamic chamber, just forward of the air bypass solenoid, little 2-pin plug on it...

Look for the OTHER chart

Although, is that just the hot-start thermal "switch"? Hmm, back to the books

 

OK, here it is- page 4A-81, it's a thermister, so you can "do it" with a meter...