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Justinb
Justin Broome

(26 posts)

Registered:
06/10/2009 12:33PM

Main British Car:


Loss of Power on 4.2L V8 with 14cux efi
Posted by: Justinb
Date: January 03, 2015 11:47AM

Hi guys,

After about 10 minutes of driving I start to experience a "stutter" and loss of power while driving. At times I can't get the car above 40mph and then out of nowhere the problem vanishes and I'm doing 70 without an issue.

It only seems to happen after 10-15 minutes of driving, but when it starts happening, it keeps going through this up and down cycle for the rest of any trip. I don't recall hearing any backfiring, just loss of power.

Car specs:
Rover 4.2L V8
14cux efi
proper ecu for a 4.2L V8
Early fuel tank
Late sender
Mr. Gasket fuel pump for pulling fuel from the tank and into a fuel filter (the kind that looks like an oil filter)
MSD Fuel Injection pump


So far I've checked:
Distributor is tight and not moving. (Buick 215 Distributor with electronic ignition)

Coil is tight and not vibrating (MSD Coil)

I removed my smaller fuel pump and filter and replaced them with an inline filter before the fuel injection pump. This made no change with the loss of power situation, so I feel I effectively ruled out the smaller pump and filter.

In my mind, these are the possible issues:
Electronic ignition is having an issue
Fuel pressure regulator is having an issue
MSD Fuel Pump
ECU is having an issue
Tune resistor failing?

Anything else that could be an issue?

Are there ways of testing these items individually? I have a 3.9L 14cux brain that I know is in good condition and works well, so I will try swapping that in.

Thanks!
Justin


afirover
Gary Grey

(5 posts)

Registered:
01/14/2015 06:49PM

Main British Car:


Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: afirover
Date: January 16, 2015 04:49PM

Do you have a vss ?
tps ?
any codes present ?


ptschram
PT Schram
Churubusco, IN
(6 posts)

Registered:
11/24/2013 05:32AM

Main British Car:
'67 through 2001 Land Rovers.

Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: ptschram
Date: January 18, 2015 12:20PM

Does it do this when at idle?

The time frame is curious to me as this could be due to the ECU going into closed loop mode and something not providing a consistent input to the ECU and the ECU thinking it is reasonable information.

It could be EMS or it oculd be mechanical and you have a vacuum leak that only becomes apparnet when the engine warms up and something expands and begins to leak engine vacuum.


rkas3882
randy kassed

(84 posts)

Registered:
05/30/2010 09:14AM

Main British Car:
1973 mgb gt rover 3.5

Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: rkas3882
Date: January 29, 2015 09:13PM

how about oxygen sensors? Are you running them? When I had one go bad on a couple of occasions it acted like this, might be worth a shot.


Justinb
Justin Broome

(26 posts)

Registered:
06/10/2009 12:33PM

Main British Car:


Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: Justinb
Date: July 06, 2015 05:47PM

I know its been a while, but I'm still battling this demon.

Here's what I've done:
Replaced the fuel pump with a walbro high pressure pump mounted close to the tank
Replaced the inline fuel filter before the pump
Drained and checked tank for rust and crap (none present)
Replaced the coil with a known good working coil
Replaced the ecu with a known good working ecu
Adjusted tps position for proper voltage
Adjusted MAF for proper voltage

None of that had any impact
The only time I got this thing to even throw a code was when I had the TPS adjusted out of whack and it threw a code 17 (TPS)

Beyond that, nothing freaks this thing out. I've unplugged the MAF while running, no codes. Unplugged an O2 sensor, no codes. I'm running with no tune resistor right now, and that's not having any impact on anything either.

I'm on a staycation this week so I went and bought an inline fuel pressure gauge and wired it in a few inches before the fuel rail.

The car starts and runs, but it only has around 28-30psi fuel pressure. Even when I hit the throttle it only has 28-30psi of fuel pressure.

While still at idle and the engine not really warmed up at all, I disconnected the vacuum hose to the fuel pressure regulator, plugged it so the car wouldn't stall, and fuel pressure immediate jumped to 38psi.

Is this normal behavior or have I discovered my problem?


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: BlownMGB-V8
Date: July 07, 2015 11:19AM

Rail pressure is referenced to intake pressure so the differential remains constant. Manifold pressure is low at idle and the rail pressure should follow it. The use of a manifold absolute pressure gage instead of a vacuum gage makes it easier to understand.

Jim


rkas3882
randy kassed

(84 posts)

Registered:
05/30/2010 09:14AM

Main British Car:
1973 mgb gt rover 3.5

Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: rkas3882
Date: July 08, 2015 02:12AM

Have you tried rover gauge? Mine was doing this same thing whenever I had an O2 sensor go bad...it should throw a code but the rover gauge will allow real time observation of the O2 sensors cycling. Can you try some known good ones?



DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: DiDueColpi
Date: July 08, 2015 12:34PM

Have you tried a different ignition module?
If you're using the Pertronics setup it won't be well matched to your MSD coil and will overheat.
Other conversions have issues with the same thing, along with rotor phasing problems.
Try closing up your plug gaps to 25 thou. If the situation changes or improves, you have an ignition problem for sure

Live like you mean it
Fred


Justinb
Justin Broome

(26 posts)

Registered:
06/10/2009 12:33PM

Main British Car:


Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: Justinb
Date: July 09, 2015 10:15PM

I did a little bit of driving tonight and have more of an update.

When I first started up the car the fuel pressure was around 30psi and was rock solid. When I blip the throttle it jumps but then settles back around 30psi.

After about 10-15 minutes of driving the car started acting up so I popped the hood and checked my fuel pressure gauge again. The needle was moving (almost in slow motion waves) between 10 and 20 psi. Even when I pulled the vacuum line it was still moving in waves, just at slightly higher pressures. When I revved it up the pressure would jump, but never really getting to 30psi. Always just bouncing around.

RPM's were steady the entire time I was checking out the gauge after the issue started. Even when I revved the car by hand, the rpm's were smooth and steady with no backfiring or stumbling.

Side note: I just changed to the 3900 ohm tune resistor (from none before) and the idle jumped 200 rpm's and also the car is now backfiring/popping when I let off the gas. (Vacuum leak or too far advanced?)


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: BlownMGB-V8
Date: July 10, 2015 11:07AM

Sounds to me like a bad fuel pump. Either that or the regulator but the pump would be the first suspect.

Jim


Justinb
Justin Broome

(26 posts)

Registered:
06/10/2009 12:33PM

Main British Car:


Re: Loss of Power on 4.2L V8 with 14cux efi
Posted by: Justinb
Date: July 31, 2015 10:12AM

Knock on wood, but I think I've got it resolved, and I think it was a handful of small issues.

1) Had some melty wires running past the passenger side exhaust manifold and down to the starter. That seemed to be messing with voltage to the entire car. I fixed the bad wires and ran them through a heat shield sleeve.

2) When problem was happening, voltage to the fuel pump was fluctuating, never getting even to 11.5 volts so that wasn't helping either. I ran a 12 ga wire directly from the battery and installed a relay feeding the fuel pump. Voltage is now good.

3) When I installed a new fuel pump and new filter before the fuel pump I made a hard turn in the rubber hose and it was kinking up some. I bent some lines and removed the kink from the hose.

4) I had one good (not great) drive into work and on the way home it went back to running horribly. When I got home I got down next to the car and I could hear the fuel pump whining. Feeling at a loss for anything else to do, I hit the bottom of the fuel tank (early mgb tank with the built in sender) right beneath the pickup, and about 5 seconds later the pump wasn't whining as much. I checked the fuel pressure gauge up front and it was now holding steady.

5) This morning I swapped the lines so the return line goes through the built in pickup and the feed line is through the late sender. Not a single loss of power the entire 35 mile trip to work.

Still need to give it a few more drives, but it was a night and day difference driving it this morning compared to last night.

Does anyone know of a way to get to the screen on the built in pickup of the early tank? LONG screwdriver?


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