Down on power.

03blackvert

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We are the proud new owners of a 1992 fox body. I might post more about that later. For now, I have a drivability issue I'm trying to do some diagnosis on myself before getting it off to the mechanic next week for some TLC.

Symptom: Low power above maybe 1,500 RPM.

Taking off from a start with moderate throttle it pulls ok until about 2,000 RPM then the power just sort of levels off at what feels like 4 cylinder acceleration. While driving at 35-45, I can push the throttle right into the floor and at first nothing happens. No acceleration, no downshift. After 1-2 seconds, very slowly, it revs a small amount and eventually downshifts....but the movement forward is weak. Interestingly, if very easy on the throttle, maybe 10-20%, it seems to move forward as expected for that amount of throttle.

No misfires, no backfires. I'm no expert to listening to engines but it sounds "good" no matter the RPM range. I did hook up a vacuum gage to the tree, first by the speed control port then again by the AC port. At idle it's 18in. 2,000 RPM sends it to about 22in. 3,000 gets it to just under 25in. The needle is steady as long as I keep my foot in the same spot on the gas. I know that's not a complete diagnosis, but I think it doesn't have a major vacuum leak.

I was thinking maybe fuel filter, but if it was being starved due to the filter, pump, or injectors it would probably lean out and misfire or backfire, wouldn't it?

Anything else I should armchair diagnose in my driveway?
 

MG0h3

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Go borrow or buy a fuel pressure gauge kit from whatever parts store.

Since your car is auto, you can add load by power braking.


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Bullitt1448

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Start with the basics, compression test, leak down test, throw some new plugs in it while you have them out.
 

03blackvert

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I am now quite confused, but I'm hoping one of you guys can help me figure it out.

I was able to borrow a fuel injection test kit from AutoZone with the right gauge in it. I found a video online that showed connecting to the Schrader valve on the chrome tube near the oil fill. I connected it all up, and got a reading of.....0 psi with the key just in on. So I started it. Still 0 psi.

Figured I must not know how to plug it in, so I took it all apart, put it all back together, reconnected it and tried the test again. Still 0 psi, needle doesn't budge.

I was worried the gauge might be bad since it's a loner tool, so I connected it to a tire just to check and it went right to a correct PSI reading.

Any idea how a car could have no pressure at the test port, but still run?
 

03blackvert

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Ok, turns out I would make a lousy mechanic!

I thought to try just pushing in the schrader valve (engine off) and be sure some fuel came out. It did. I looked at my fitting, no fuel in it. So I put it back on and tightened some more. I think the threads are so crusty I thought it had bottomed out but it was just hard to turn.

The pressure came up real slow. It took about 35 seconds of it slowing rising for it to finally get to 32 PSI. I'm thinking the pressure should be up in much less time, like around 5 seconds?

If the pressure is good but it comes up slow, does that mean fuel filter?
 

2003RedfireVert

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That seems long. Did you start it to see what the pressure does?

It should build pressure within a few seconds.

This seems to be at least one issue.
 

MG0h3

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Fuel pressure should come right up to at least 30 with the first key cycle, cycle off/on a second time and should see 40. I think stock on those was 39psi.

You need to do it running and more importantly, when the failure is occurring under load.

And no, not necessarily the filter but possibly.

When diagnosing a driveability problem, you always have to ensure the “tune up” components are good to go.

Plugs
Air filter
Fuel filter

Let’s assume the plugs are fine because this doesn’t seem like a misfire to me.

But do what I said above, then start the car. Snap the throttle, hold it at 2500-3000rpm and record fuel pressure.

If you have a fuel delivery problem, first thing to eliminate is the filter.


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03blackvert

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My buddy with a 4 post lift was free, and the local auto parts happened to have a genuine Motorcraft filter in stock for $18 so we took a swing at replacing it. The videos online said the new filter should come with new plastic clips, and mine did not. Harumph. We pressed forward though to see what difference it made.

The answer is, a bit! It now has more power, even can (just barely) chirp the wheels from a start. Part of the problem was absolutely the fuel filter. It's still not where it should be though, so there's more to do. I'll probably let further diagnostics be done by a real mechanic though, I'm taking it in this week for some other overdue work.

Thanks a bunch guys!
 

TK1299

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How old is the fuel that’s in it? You should also smoke test it for vacuum leaks, regardless of if that’s the issue or not. These cars are vacuum leak champs and hard to knock out. Vacuum leaks will definitely cause low power issues.
 

03blackvert

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It was delivered (bought online) on E, so it's got completely fresh fuel in it.

I plan to have a mechanic smoke test the vacuum system this week, I don't have the right stuff to do that. Pretty sure there is also a trans problem, it doesn't shift right. I'm hoping it doesn't need a full rebuild and that a good fluid flush will do it, but we'll see what the mechanic says.

We expected some mechanical issues given the age and the story we got with the car...but it was in such amazing condition otherwise (and basically fully stock) we took our chances.
 

MG0h3

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Whoa. Well being setup on E changes everything.

It should have aftermarket pump(s), injectors, regulator and most likely lines and rails.

Can’t imagine any stock Fox stuff would flow enough for E.




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gimmie11s

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Fuel pressure should come right up to at least 30 with the first key cycle, cycle off/on a second time and should see 40. I think stock on those was 39psi.

You need to do it running and more importantly, when the failure is occurring under load.

And no, not necessarily the filter but possibly.

When diagnosing a driveability problem, you always have to ensure the “tune up” components are good to go.

Plugs
Air filter
Fuel filter

Let’s assume the plugs are fine because this doesn’t seem like a misfire to me.

But do what I said above, then start the car. Snap the throttle, hold it at 2500-3000rpm and record fuel pressure.

If you have a fuel delivery problem, first thing to eliminate is the filter.


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This


I’ll add ignition components… distributor/cap ensure timing is advancing the way it’s supposed to.

No timing advance will make the car feel exactly as you describe.


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DSG2003Mach1

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Whoa. Well being setup on E changes everything.

It should have aftermarket pump(s), injectors, regulator and most likely lines and rails.

Can’t imagine any stock Fox stuff would flow enough for E.




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Pretty sure he meant Empty tank
 

MG0h3

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railroad

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Set your base timing to 14* adv. 91-93 octane gasoline.
I had no problem with 16* adv on 93.
 

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