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Discussion Starter · #1 ·
Car has severe lack of power and almost "sputters" (stalling type noise) when trying to gas it or go up hills especially in low RPMs. Otherwise runs alright with no major hiccups, just no power anymore. The feeling is like 100% power when cold (first 15mins) then down to like 40% when warm/hot.

For the first 10-15 minutes of driving (hard/soft) the car runs GREAT. After about 20mins the oil pressure gauge starts dropping (cold=needle in middle, warm=1/3 to lower portion of gauge) and stays with in the same levels depending on the RPMs.

History:
About a month ago after driving long distance in hot weather parked car for a few hours then went to start car and it sputtered (had trouble keeping engine alive without giving it gas) but after a minute or two of revving, car was normal.

I then replaced the fuel pump and the starting issues has never happened again, but the car still has the lack of power issue when WARM.

Also car consumes (possible small non-dripping leak) about 1 quart of oil a month/2months. And the gasket where the head meets the block looks a little wet (oil), but has been like that for years.

I have done pretty good maintenance over the years including spark plugs, fuel filter, ths usual, along with cleaning the MAF, IAC, and throttle body. Had the AOD tranny rebuilt 2 years ago.

Car is pretty much stock at 155k miles. It is a 1991 Mustang LX 5.0L with k/N filter, shorty headers, hi-flow h-pipe with CATS, flowmaster muffler, shorty pulleys

I could use a new radiator (inside corrosion), but the temps never seem to reach more than 200 degress (needle at middle of gauge) or maybe slightly higher when driving in upper 90F weather.

I would appreciate any feedback or recommendations because I would love to keep this car alive and still possibly do some more modifications once I have the problem fixed... I would love to.

I fear it might be something bad like a head gasket or an engine rebuild. but hopefully it could be something as minor as vacuum house or an oil pump or too much heat (not enough heat dissipation from raditor).

Thanks for any help that is offered.
 

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Have you tried driving at night to see if it maintains more power longer because it is cooler at night?

Have you tried pulling any error codes ?
 

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When cold your running a richer fuel map. When the car is warm it is leaning out which is the reason for the power drop. Pull #7 plug after you drive about 10 minutes and it feels weak on power. You should see a lean plug condition.
 

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Discussion Starter · #4 ·
If it that is the case, which I believe it to be (haven't had a chance to check yet), then what is the next step to allievate the problem?

Why would it be using a leaner fuel map when cold versus warm (normally about 20mins worth of freeway driving will do it)?

And how/what can I do to make it inject more fuel or figure out why it is not injecting enough fuel when it warms too much?

FYI: i haven't pulled codes... I don't have tools for it, but I will probably take it by a shop when I have the time.

Thanks for your help too!
 

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I don't have any good advice on your power problem, but I just wanted you and everyone else to know that as of 3 days ago, my oil pressure needle does exactly what yours does. starts off in the middle when cold, then drops when warms up. Now that would make sense if the oil was changing in temp and viscosity, and bearings are heating up and expanding etc, but mine is funky where it will basically do whatever it wants.. sometimes go up to normal, sometimes hang low, very inconsistent.

This happened in my old 89 as well, and I replaced the sender unit as I heard they are bogus, but same results. I am going to put in a mech. gauge and see exactly whats happening

Matt
 

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Discussion Starter · #6 ·
you are talking mechanical oil pressure gauge right? (just clarifying)
 

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Yes, and today I installed a mechanical autometer gauge with a -4 line, and my findings do concern me.
When the motor is cold I'm pulling 60 psi of pressure, and when its hot I'm pulling 42-50 psi(@ 2000 RPM), but when I'm idling it will dip as low as 23-25 psi.

If I'm not mistaken shouldn't I be pulling 45 psi at idle and closer to 70 @ 2000 rpm?
Although those #s may be when the motor has signifigant less miles on it.

Regards

Matt
 

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speedytang said:
When cold your running a richer fuel map. When the car is warm it is leaning out which is the reason for the power drop. Pull #7 plug after you drive about 10 minutes and it feels weak on power. You should see a lean plug condition.
what do you mean spark plug how do i now if it is lean what color of the tip or something?
 

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aleborjas said:
what do you mean spark plug how do i now if it is lean what color of the tip or something?
White = lean; golden tan is good; and black indicates rich.
DWCLS, you only need a jumper wire to scan. The check engine light will flash out the codes. Refer to the tech section on this site. I think you will find your problem to be either the ACT or the ECT sensors.
 

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Discussion Starter · #10 ·
So I put in a new "direct-fit" Summit aluminum radiator (which wasn't direct fit at all. everything including brackets and holes had to be adjusted/redrilled) and a new 180 themostat. The Engine is cool as ice and runs much stonger (i think my computer was retarding my engine when it got too hot or became too lean, not sure).

I think meangreen might have made a good point about a faulty ECT (or ACT) sensor mixed with a inefficient corroded 15 year old radiator. But the car has always run a bit on the lean side, so I might pull the o2 sensors too. And I am going to check the TPS to make sure it was not fouling up when hot.

And i bet my valve seals are leaking a bit, but I won't worry about those until the engine dies completely (hoping I get at least another yeat out of it)

Anyhow the assumed to be not-so-accurate temperature gauge now sits pretty much rock solid on the first line in the gauge where as before it would float from the second bar to half way on the guage or above... huge difference!

So I think my power loss probably had/has to do with bad sensors or something... because it still feels just a little tiny bit weak when running for a while, but NO where near what it was at before.

And the 180 degree thermostat... I can say I love it (combined with a nice aluminum radiator that can keep the coolant cool :) )

Anyone have anything else to add? :)

thanks guys!
 

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Discussion Starter · #11 ·
:)
so here is the scoop on the performance issue:
Mass carbon build up in the engine + advanced timing approx 15 degrees using 87 octane = poor performance.... oops

after finding that out.... not even 91 octane could help.

but, it wasn't my fault...
I bought the car used at 89k miles in '98 (timing was previously set to 15 degrees. I did not know) and it drove just fine (so i thought) but over the past four years the engine started to perform poorly when it got really hot. It only made some funny noises after driving it for a period of time and then pushing it hard. Found out the funny noise was detonation... though sounded like stalling noise.

I had the engine checked out again... Leakdown and compression tests were withing range. PCV filter changed (now the oil gauge reads normal again, versus dropping, thanks Joel 5.0), Seafoam engine cleaner run through the fuel, vaccum lines, crankcase, etc....(smoked a friggin thunder cloud out the back for 5mins)... and lastly, timing set back to 10 degrees for test purposes.

Also replaced the balancer, as it had slipped (which made it impossible to read the original timing :) )

Car runs like a dream! I didn't know the 5.0L engines were supposed to perform this well :)

I think I will try to bump the timing up to 12-13 and run 91 octane. I don't think it will detonate any more as all of the carbon is now pretty much burnt out. I believe that the carbon build up was causing the severe detonation after the car had been driving in excess of 30mins (read: hot)...

anyways... thanks for the help everyone for getting me motivated to fix this.

any other thoughts?
 

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dwcls said:
FYI: i haven't pulled codes... I don't have tools for it, but I will probably take it by a shop when I have the time.
Glad to hear she's running better. These engines will last a good long time, if maintained halfway decently.
Not sure, but I think you can clean the ECT, it could be scaled up.
You don't need anything other than a jumper wire, to scan it for codes.
 

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i had similar problem once.it turned out to be just the ignition mudule
 
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