Rob the plumber

I was using a fairly large, junkyard check valve to the map sensor on my Turbo Jeep project. It was only making about 4 psi, but ran excellent, and made pretty good power throughout the powerband. Just for kicks, I plugged the check valve to see how much boost I am losing. It now makes 8 psi with a lot less lag. Unfortunately, the engine falls flat at about 3 psi. It makes no more power. I get no check engine light, and the engine still runs very smooth. It pulls nice until the gauge hits about 3 psi, then it actually loses power.

I am guessing that the MAP is sending the PCM numbers that are confusing it, or the spark is possibly blowing out. I doubt it is an ignition problem though. I have an MSD 6AL-BTM, with plugs gapped at .030. For fuel I am using a 255 Walbro pump and a 12/1 FMU, so I think I have enough fuel too.

I think I should experiment with one little fish tank check valve and see what happens. What do you guys think of products like the MAP clamp http://www.gcsrts.com/dasmopar/clamp.asp or a FuelMapper: http://www.turbosystem.com/auto/engine/fuelmapper.htm I think these products use a type of Zener diode to limit the output voltage of the MAP sensor. It would be nice not to dump that nicely compressed air to atmosphere......

Rob the plumber

I did some testing, and my engine is falling flat right when the MAP sensor hits 5 volts. I installed two fish tank check valves, but it loses a lot of boost that way, and the MAP still sees 5 volts, it just takes a little longer.
I think I should try one of these MAP voltage limiting devices.

HiProfile

What sort of check valves are you using, and how are you setting them up?? The check valve on a Honda is used to keep the MAP from seeing any boost, since >1psi = CEL. The missing link setup that Honda people use is a check valve with a very tiny hole between the valve and the MAP to keep the valve from shutting when going from very high vacuum to zero vacuum, as well as not pressurizing the MAP due to any little boost leakage.

I could take a guess at how you're using the valves, but I honestly have no idea. The MSD will retard ignition, but without setting the timing retard on a dyno or by monitoring knock, it may not be helping much at all. As for your 12:1 FMU... you can't guess it's getting enough fuel, you have to PRAY its getting the right amount. Without checking the AFR's with a wideband, your only way of checking is via reading the plugs, which only gives you one rpm/psi point to check - and the inconsistent FMU makes that nearly impossible.

Since the MSD 6AL-BTM has it's own sensor, you need to make the checkvalves work like they do in a Missing Link. With a proper setup, your stock MAP should only see 14.5psi absolute for any amount of boost (basicly never see boost, but see vacuum when present), while not leaking any boost from the manifold.

BTW there's a chance your ECU won't display a CEL for boost, rather it retards timing severly due to what it thinks is a MAP malfunction. Even with an undersized turbo, 8psi will always be more powerful than 4psi unless you are detonating, have way too much ignition retard, or are very rich or lean.

Rob the plumber

I have the check valves set up to bleed boost off to atmosphere before the MAP sensor. I set the retard on the MSD box at 1.5 degrees of timing per pound of boost. The 12/1 FMU with a Walbro GSS340 pump, along with the MSD settings are what I have always used on my 5.0L Mustangs back in the day. Those were mass air cars, so I did not have to deal with tricking the MAP sensor.
I ordered one of those MAP voltage "clamps". along with a wideband to be sure my a/f ratios are in the safe range.

Skaterdudz02

I'm using the check valve and FMU setup on mine again (3 tetras and 12/1 FMU) (chipped ECU took a ----). I also have a wideband, and usually anything after 5 lbs it runs lean at about 15.5-17 which sucks. Anything 5lbs or under it's usually in the 12-12.5 range. I always thought to believe that this setup there wasn't enough fuel getting into the cylinders hence having bigger injectors would probably do the trick. Can anyone confirm that? Even if it were a 50cc difference I'm sure it would be able to handle more fuel that way?

t_cel_t

use real management

J-MAN

what year jeep? must be somewhat new since it has a ECM. i would say junk the FMU but i have never doen a turbo on a jeep before and i can't imagine there being a whole lot of other options. post more info on your setup. and im thinking that 1.5* per psi of boost might be a little much, i usually try and use .5* per psi

Rob the plumber

If you have a stock fuel pump, that is your problem. The flow of a fuel pump is greatly reduced at elevated pressures. You need a high volume and high pressure pump to delivery enough fuel with an FMU.


My Wrangler is a 1995, 2.5L. Stock engine, Ford 19# injectors, ported 4.0L throttle body, T3 blowing through a Saab intercooler. Apart from a complete standalone, the FMU is about my only option. This was also more of a low-budget winter project before the V8 went it. I just wanted to see if I could do it, and now I like it. I am also not looking to squeeze all the available power out of this thing either. I want to keep the boost low, tune it conservatively and make it reliable. I would like to shoot for a total timing of about 20-22 degrees under boost, but I cannot find any information on the parameters of the Jeep ECM.

Skaterdudz02

I thought it was that when I first did my turbo setup, but I did switch over to a chipped ECU and it ran 8lbs no problem and didn't have any fuel issues. (11.5-12.5 on all bands). Spiker ran a stock fuel pump on his 10lb turbo setup. Anyone else have same issues that a walbro fuel pump solved the issue?

Rob the plumber

Running larger injectors and a custom tune is "easier" on the fuel system. I ran the same GSS340 pump and 42lb injectors with no FMU in my Mustang and made about 600 horsepower. I think I should have plenty of pump for my turbo Jeep that makes maybe 200 hp if it were lucky.