Lack of Power on WOT - Fuel Pressure Specs? Other?

[Mike1261]

Since owning my 03 GT, I've found that when I punch it, it feels like there's lots of noise and no acceleration. It's faster to accelerate with 1/3-2/3 throttle.

Finally felt like messing with the car again so decided to figure out what is causing it.

I just redid the fuel system in my 06 BMW so figured I'd start there on the GT too... But I don't know what correct behavior is, so hoping someone here might comment on these readings:

All readings taken Parked on Driveway in Neutral. No load.

Cold Engine:

30 psi idle
Blip WOT throttle: Shoots to 40/45 then drops to 25 when releasing then settles back at 30/idle

Hot Engine:

35 psi idle
Blip WOT throttle: 35-40 psi
Release Throttle: Back to 35

With hot Engine, unplugging the vacuum line to the pressure regulator has the pressure sit steady at 45psi. I've seen videos where doing this causes it to sit at 60 psi so I'm wondering if my pump isn't able to maintain pressure?

I'm also open to suggestions on what else to check. I have new spark plugs that I haven't gotten around to it yet, but I pulled one or two of the most accessible plugs and they actually look fine.

I also checked engine vacuum and it's right at 20.

Then our sunny Florida weather turned to a downpour in 30 seconds flat so here I am

Also: New clutch and no slippage so it's not the engine overpowering a bad clutch causing the problem either.

Thanks!

 

[ttocs]

what mods are done to the car?

 

[Mike1261]

what mods are done to the car?

Bone stock. 165,000 miles. TR3650.

 

[Silver95bird]

I don't see anything wrong with that pressure. at WOT it should approach 40 and not dip until you let off.

what rpm level is it OK to? where does it drop off at?

any obd2 codes?

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[96blak54]

Engine cold running compared to engine hot running.

Does the car seem to run strong when the engine is cold? But when at temperature, it falls off?

 

[ttocs]

is the stock temp gauge at the end of the gauge when it does this?

 

[Mike1261]

Haven't driven the car since the convertible top motor crapped out in Jan so just going from memory:

No overheating issues, temp gauge always where it should be.

No specific RPM, entirely related to throttle position.

And with 160k miles and no money to rebuild the engine, I don't get on the gas when engine isn't warmed up so no idea if it's related to engine temp cold/hot. I'll have to go out and test it tomorrow.

It's very subtle. No stumbling or stuttering or sudden loss of power. Just an absence of acceleration but lots of noise and RPM and a wife that wonders why the 1987 Corolla seems to be accelerating faster than us.

It FEELS like a slipping clutch... Engine RPM going up but the car not really going. So much so that I've tested for clutch slippage many times just to make sure it's not it.

I thought maybe the air filter was clogged and the engine couldn't breathe, but I cleaned the K&N and left it off entirely as well.

No CEL on.

 

[Silver95bird]

Just a shot in the dark, but has the MAF ever been cleaned?

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[ttocs]

it almost sounds like its going int a limp mode if that is possible

 

[Mike1261]

Just a shot in the dark, but has the MAF ever been cleaned?

I just cleaned it though, had some MAF cleaner kicking around. Looked immaculate when I opened it up, surprisingly. But you never know, maybe there's a thin grease film over it? It started raining before I got to put it back together, will report back if it made a difference!

it almost sounds like its going int a limp mode if that is possible

Doubt it. If Limp Home on Mustangs is the same as my old 88 Camaro, then probably not the case. It has more power than Limp mode, and power returns to "normal" if you stop WOT'ing it. My Camaro would stay in Limp mode, it wouldn't go back and forth between it.

ETA: I wonder what a compression test would show. Maybe the engine's just toast. Need to see how to change spark plugs on these 4.6's and I'll run a compression test while I do the plugs. Looking at the coils, they appear to be blocked by the fuel rail, so presumably I need fuel injector O-rings on hand if I need to pull up the fuel rail to do the plugs. I'll youtube it or read the Haynes manual. At least they're not down between the headers like on a 350 Chevy lol.

 

[96blak54]

Force the car into limp mode by unplugging the maf sensor then drive the car. Maf sensor make a car do strange things.

 

[ttocs]

I had a strange temp related issues with mine last year we discovered during tuning that could be related here. The problem I found was that after the motor was hot, the harness for the TPS sensor would get loose and then the tps would start to drift and show 4-8% throttle when I wasn't doing anything. Try getting the motor warm and then go grab/pull/jiggle all the wire harness connections you can find and see if it stumbles or if any number change.

 

[96blak54]

^ good idea

 

[Mike1261]

Cylinders 5678 have 175-190psi compression so that's better than I expected lol.

5678 coils all test within spec and identical to each other.

5678 spark plugs look fine. Replaced with NGK.

Pending weather I'll do 1234 this evening. I won't have any dielectric grease until tomorrow so won't put the coils back in today.

I'm hoping for a weak coil on the 1234 bank lol. Hopefully compression will be consistent across the board as it'll give me hope for this engine yet.

Once it's all back together again I'll road test and see if the MAF cleaning did anything and play with the other suggestions above as well.