Car stumbles under WOT at 4200-4500 rpms. I'm stumped....

[jwm3786]

Ok my car consistently falls flat on its face under wot at 4200-4500 rpms. It almost makes a backfire type noise but no black smoke. I've done several things I'll explain what I've done and my setup.

I have an 03 cobra with a whipple 2.3 with 2.75 pulley, fore fuel system (rails, hat, 2 gt pumps, push lock hose) dual fpdm setup an a tune by Mcnews automotive.

Started after I did the initial upgrades of the fuel system, I have tr7ix plugs gapped at .032 I believe. I took I back to the tuner and they said it wasn't the tune, we will see I guess. I data logged a little and saw that when this happens I lose all fuel pressure. We rechecked all the connections to the pump and re soldered them to be sure. I ran a new frps to see if that was it and although the old one seemed fine, it still did the same thing with the new one. I've tried smaller gaps on the plugs, no success.

So I'm wondering if anyone has experience this or has an idea of what else to check? I'm stumped.

 

[jwm3786]

Ttt, anyone? Am I missing anything obvious?

 

[46stang]

I would data log TP sensor voltage and the rpm signal. I have seen and experienced both of those things cause my car to buck and jerk. If the TP sensor is bad when it happens you will see the voltage drop down and back up even though your foot is still planted. The screwy thing is the sensor is still reading a voltage so it won't set a code.
The only other thing I can think of is to also check all your wires going to the pumps and make sure none of them are shorting out or loose.

 

[Rossim22]

Have you checked all your plugs and inspected the straps? Otherwise sounds like a possible tuning issue in that rpm window

 

[TRBO VNM]

what happens past this rpm? does it seem to just fall on its face during that rpm and then come back without issue? mine did that at 3500 and it turned out being my MAF. I was using a gt500 sensor and an adapter harness from vpm. I haven't confirmed if it was the harness or the sensor, but I put a sct ba3000 in and I am fine now.

on the dyno we checked the power wire to the coils and they were losing 3 volts at that rpm.

 

[jwm3786]

I have adjusted the tps and it was at a .95 volts but I'll check again. Ya never know.

The spark plugs have always looked very clean despite this issue....

After this rpm if I stay in it it will pick back up and go. I just don't like to do it because I'm afraid I will break something given that I destroyed my 97 cobra motor. But I have the exact sct meter you switched to. I'm going to talk to the tuner tomorrow hopefully. Maybe he will see something. I'll keep everyone posted.

 

[SVT_Troy]

Check coils. Try to find someone local to borrow for a test drive and see if it still stumbles. Odds are its either spark or fuel related if you have normal vacuum, smooth idle, normal gas mileage and no loss of power other than that rpm range. Monitor your fuel pressure under wot to make sure it's staying consistent under wot

 

[Tifosi2003GT]

bump..any update?

 

[CanuckSnake]

One fuel pump dead?

 

[TRBO VNM]

Check coils. Try to find someone local to borrow for a test drive and see if it still stumbles. Odds are its either spark or fuel related if you have normal vacuum, smooth idle, normal gas mileage and no loss of power other than that rpm range. Monitor your fuel pressure under wot to make sure it's staying consistent under wot

You would think right?

Mine drove perfect for a long while, dyno'ed perfect and everything except at 3500 it would fall on it's face. If I drove through it, it would pick right up and keep going just like the OP is experiencing. I had the WOTbox and removed it since I didn't care to have it on my current setup and though my splicing was fu-bared or something because when I redid the connection the symptoms changed for a couple of drives and came right back. Finally it started doing it under very light throttle, like chugging along. Give it a little load and it was fine, WOT was fine until 3500 and bam. then would come back just above it.

Just for the hell of it because Jon has seen issues with the gt500 sensors in the past and after talking through some other things like the tps and other sensors, figured, hey, got a stock maf and mafia sitting here so I put that on, loaded a tune for it and beat the crap out of the car for 25 min and no more issues. JLT and SCT to the rescue. Hope to retune on the street this weekend.

Anyway, OP, something to look at, at least. And with the TPS, you might be able to log it with car off and with your hand pull the throttle blade WOT and see what the voltages to throughout the range. Maybe it drops at a certain point.

 

[jwm3786]

Hey guys,
All good suggestions and I will check all of it. Work has been crzy lately so I'm hoping to do some data logging tomorrow night. If not Sunday will be my next chance. Hope to have this all sorted out before cruise week. I'll definitely keep everyone posted.

 

[jwm3786]

http://s1226.photobucket.com/albums/ee420/jwm3786/?action=view&current=CobraDatalog1.jpg


That is one data log. Still new at this. But I checked the TPS it is at .99 volts I moved it back to .98. Thats good. The plugs looked good. The only thing I saw was this fuel pressure drop to 23psi when I went WOT in third gear. Any ideas or any other info you gurus would like to see? Could it be alternator? I didnt get a chance to check that as I thought about that afterwards.

 

[theunseenclass]

yea it could be your alt.

 

[46stang]

And with the TPS, you might be able to log it with car off and with your hand pull the throttle blade WOT and see what the voltages to throughout the range. Maybe it drops at a certain point.

Like he said^^^^^^^

Your problem isn't happening at idle so checking the tp voltage at idle isn't telling you anything. turn the key on, while recording on the data logger, grab the throttle cable and slowly open the throttle until it is wide open. Then look at the recording. the voltage should gradually increase until it is maxed. If you see any downward spikes the TP sensor is likely bad. You an also record it when the car cuts out and see if it spikes down.

 

[jwm3786]

Like he said^^^^^^^

Your problem isn't happening at idle so checking the tp voltage at idle isn't telling you anything. turn the key on, while recording on the data logger, grab the throttle cable and slowly open the throttle until it is wide open. Then look at the recording. the voltage should gradually increase until it is maxed. If you see any downward spikes the TP sensor is likely bad. You an also record it when the car cuts out and see if it spikes down.

Did that, forgot to mention. But it gradually increased from .99 to 4.72 as i opened the throttle slowly.

 

[46stang]

does it only cut out between 4200-4500 rpm when going wot or does it do it all the time at that rpm?

 

[jwm3786]

It only cuts under WOT as far as I know. 1st and 2nd gear through that rpm range are fine when its less than WOT. Im going to try and datalog the alternator voltage. I forgot to mention I have an SCT BA3000 MAF. So I dont think its that. Im starting to think its the tune. Maybe time to get a second opinion.

 

[Tifosi2003GT]

how long have you been running this set-up and when did your problem start?

 

[jwm3786]

how long have you been running this set-up and when did your problem start?

Well, I got the car last April, bought all the parts in may and put them on in may. Had it tuned and as far as I know it has done it ever since. I didnt get a chance to work on or drive the car last year more than maybe a couple hundred miles and most of it was highway to and from the beach. Then put it away for the winter. Now I have a chance to work on it this year and Im trying to figure this out. But as I said, as far as I know ever since we put all the parts on.

The whole setup is:
Cooling mod
Heat exchanger
Whipple 2.3
Billetflow SBTB
JLT CAI
Fore dual returnless hat, Ford GT pumps, Fore rails, 80lb injectors
SCT BA3000
Dual FPDM setup
TR7IX plugs at .032
Tuned at McNews Automotive in Dillsburg PA


As said before Ive gone over all the wires and they all look fine......Ill try unplugging the pumps one at a time to see if one may be dead. Other than that Im stumped. Fuel pressure just drops out to 23PSI as shown in the graph

 

[Tifosi2003GT]

my car has the same mods.. minus the plugs I have tr-6 @ .028 Same exact problem. Identical to a tee