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Guest Message by DevFuse

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losing power at higher speeds

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6 replies to this topic

#1 teasdam


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Posted 14 December 2004 - 11:14 AM

this car has been getting around 23-27mpg, depending on where I've been driving...city vs. highway. Lately mpg has been more like 20-23. Also noticed that power seems to be decent at lower speeds (no access to a similar car to compare, might just be typical for this engine), but once I get to about 50-55mph acceleration takes a nose dive. Last time I checked on a relatively flat length of the highway it took me 22 seconds to get from 55mph to 65. Also can barely hold 65 if there is any incline at all. Otherwise the car runs well.

I think I've done everything that has been suggested in past threads. I rebuilt the engine in March and replaced all filters, fluids, plugs, wires, o2 sensor, etc. and timing is still good. I'm running regular gas so octane can't be too high.

Any suggestions of other things to look for? No idea how to navigate the emissions stuff but maybe that's the culprit? No sign of a vacuum leak anywhere...I went over the lines/hoses a month ago.

I'm just trying to think of what would affect power @ higher speeds but not slower. Can fuel pumps wear out gradually or is it an "all-or-nothing" component? I am getting a lot of blow-by...would lost compression produce this?

#2 swiftt


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Posted 14 December 2004 - 11:38 AM

Why are you getting a lot of blowby in a new engine? I'd look for the warranty paperwork!

Check that the timing is advancing as you accelerate. Hook up the timing light and rev the engine while watching the timing marks. The marks should advance right out of the sight window. If the advance is not working, check to ensure the vacuum line is properly connected. If your disty has a retard mechanism too, make sure it's hooked up properly and the lines are not crossed. Advance side goes to port vacuum and retard side goes to manifold vacuum.

Fuel pumps can get tired but if your car is carburated, it doesn't take much pressure to make it work. Typically, if it fails, you'll know right away. Fuel injection can suffer from a tired pump as the requirements placed on the pump are much greater. If a system is needing 60psi and the pump can only deliver 55, it will probably still start up and idle but the driveability will suffer.

You may also want to remove that big aluminum wing bolted to the rear. I know they look cool but that's a lot of drag. :grin:

#3 MorganM


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Posted 14 December 2004 - 11:56 AM

Blowby out the exhuast means you are loosing compression. As you guessed compression does impact performance. It shouldnt be THAT bad however. I suffer from a lot of blow by on my SPFI also but I can still hold 70 mph.

You can forget the vacume advance stuff.... it's an optical dizzy.

FUel starvation is a possibility. That would most likly show up as a sputter or missfire at higher RPMs. Is this just a lack of power or does it kinda sputter and missfire as you try to accelerate to 65 MPH?

Double check your spark plug gap. Often times plugs that come gapped are slightly off or completly wrong. Also the condition of your plugs can give you insight to how the combustion process is performing.

Maybe it's not your engine at all... maybe its your 3AT thats slipping at higher RPM / MPH ?

#4 Wrench Princess

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Posted 14 December 2004 - 01:43 PM

Checking the plugs in a situation like this is always a good thing. Helps you figure out what you're dealing with a lot of times. If they are all black and wet, you are getting a lot of oil in your cylanders which would explain your blow by. If they are just wet looking you may be having trouble burning all of your gas which could mean plug gap is off or coil is going out or wires aren't working properly. If they are really white and clean you could be burning water in your cylanders. And if they are really black and crusty then they are just not happy and you should change them :D . Hope that this may help you out some.

#5 snowstormer


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Posted 14 December 2004 - 05:29 PM

could be a plugged cat- i had a soob once that ran fine- no misses, just no acceleration and no power going up hills.

someone suggested a plugged cat.

brought it to an exhaust shop, they drilled a small hole in the exhaust pipe and checked back pressure- sure enough cat was plugged. they quoted me 200 to replace. i said no thanks. so they patched the hole they drilled and charged me 10 bucks for the test. i took it home and fixed it myself- gutted the cat.

no emissions testing where i live. not sure what the laws are in your neck of the woods.

#6 snowstormer


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Posted 14 December 2004 - 05:31 PM

heres another thought- if you have access to a compression tester you could test your commpression just to rule that out.

had a geo once with no power. turns out it had 50 psi on every cylinder. no wonder it had no power. did not use any oil either. go figure.

#7 teasdam


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Posted 14 December 2004 - 06:39 PM

one of the cylinders was scored bad enough that the machinist recommended I bore it out and use oversized pistons otherwise blow-by could be a problem...but this was my first attempt at a rebuild and and I didn't want to get that involved in an endeavor that I wasn't sure I could actually pull off in the first place so I didn't bother, just honed em :-p . It's a safe bet blow-by is something I did and not bad parts.

I'll take a look at the plugs again (I could use some quick release plugs on this car as often as I do this). No misfiring or anything like that, just no go. Urp, don't like the sound of the bad cat option, but that will be plan B.

Is there a quick way to test the fuel pump? I can hear it running (groaning) so it had crossed my mind that it's dying, but then again it's always done that. Looks like it would be easy to replace, I'd just rather not drop the money.

thanks for replies

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