Low On Power - Looking For Ideas

[Bornindesert]

88' 4.0l Auto 4x4 MJ - healthy engine, around 70K miles, 154K on the truck

 

Just got this Comanche back on the road this past week. First took a few short trips to break in the new gears, than a trip to the gas station followed by a quick off-road detour to check for wheel rub. Was starting to notice that it was down on power, as in struggled to hit 55 mph coming up the hill. When you floor it, starts to lug and actually looses power until you let off the pedal.

 

Passed inspection, so emmissions, evap recovery and cap all checked out with flying colors. All factory vacuum lines are in place, mostly in decent shape. Good fuel pressure, new filters, plugs and tune-up. Replaced the Cat, muffler and tail pipe. Engine runs good.

 

However, when it's under load like accelerating up a grade, the A/C moves from Vent to Defrost. That tells me it's losing vacuum under load, not sure why or what would cause that only under heavy load? At idle, coast or light load it blows out the vent again, runs smooth.

 

Starts normal, idles normal, sounds normal. can't locate an obvious vacuum leak.

 

Question - what else to look for? Could a bad O2 sensor be the cause? Dirty trottle body? It's almost like the ECU is retarding the timing.

 

Maybe I don't want to fix...because my teenage son will be driving it :dunno: a built in governor!

 

 

[1987Comanche]

  How much compression is it making?

[Bornindesert]

Haven't checked, but agree that would rule out a valve or ring issue. Thanks

[HOrnbrod]

Nothing else except a vacuum leak can cause the HVAC system to default to the defrost vents when the engine is under load. The usual culprit is a worn or broken vacuum hose where it passes through the right front bulkhead under the battery to the vacuum reservoir behind the front bumper. Or the reservoir itself could also have a bad check valve, or a leak.

 

The fix is to move the reservoir into the engine compartment, replacing it if bad (link below), or run new rubber vacuum lines out to the cannister replacing the rotten original plastic lines. This may or may not cure your lack of power under load issue, but I would get the HVAC system fixed first.

 

http://comancheclub.com/topic/16154-vacuum-reservoir-relocation/?hl=porsche

[Bornindesert]

Thanks Hornbrod, I will go through the HVAC system vacuum lines in more detail. I had all that stuff out to clean up and work in the engine bay, and thought it all went back together but may have screwed something up. The canister ball is already moved up under the coolant pressure bottle, should be easy to go through it.

[danbyrambler]

Fast & dirty vac check - - Cap all vac ports on intake except the one for the brake booster - Test drive - if still does then it's the booster - If cured then it's a one by one for the rest.

:thumbsup:

[carnuck]

MAP sensor has to be hooked up too. Sounds like a bad vacuum ball, which is allowing vacuum in near the MAP sensor, which is leaning out the mix. Assuming fuel filter is clean?

[comanchedude]

when you apply gas , like "under load" you will loose vacuum that's the normal result of extended throttle... the vacuum tank only last so long.. you will always loose vacuum under acceleration..always. and the tank need to have a one way valve on it it that's been removed or is bad...it will not hold vacuum under acceleration.

[Oyaji]

when you apply gas , like "under load" you will loose vacuum that's the normal result of extended throttle... the vacuum tank only last so long.. you will always loose vacuum under acceleration..always. and the tank need to have a one way valve on it it that's been removed or is bad...it will not hold vacuum under acceleration.

Has anyone  here found either need  to increase or benefit from increasing the volume of the vacuum reservoir, or is the stock capacity perfectly sufficient?

.

Just wondering...         

[HOrnbrod]

I added a larger reservoir when I installed cruise, but probably didn't have to. I had the stock ball tank, not the football.

[ParadiseMJ]

MAP sensor has to be hooked up too. Sounds like a bad vacuum ball, which is allowing vacuum in near the MAP sensor, which is leaning out the mix. Assuming fuel filter is clean?

 

The MAP sensor line doesn't have anything to do with the vac ball.  It ONLY goes from sensor to throttle body.  I'd look at the vac source line at the rear of the  intake manifold.  It could still pass emissions if the line was compromised and would still supply some  vacuum to the HVAC system & cruise) 

The "double" ball (football) is usually only present with cruise...which I don't have.  AFAIK it's not "double the capacity" it two separate chambers with a wall(septum) between them, and two separate in/out lines (that may or may not T together.

 

In general, even a disconnected vac ball won't affect the engine performance much.  How's the idle??

[cruiser54]

 

MAP sensor has to be hooked up too. Sounds like a bad vacuum ball, which is allowing vacuum in near the MAP sensor, which is leaning out the mix. Assuming fuel filter is clean?

 

The MAP sensor line doesn't have anything to do with the vac ball.  It ONLY goes from sensor to throttle body.  I'd look at the vac source line at the rear of the  intake manifold.  It could still pass emissions if the line was compromised and would still supply some  vacuum to the HVAC system & cruise) 

The "double" ball (football) is usually only present with cruise...which I don't have.  AFAIK it's not "double the capacity" it two separate chambers with a wall(septum) between them, and two separate in/out lines (that may or may not T together.

 

In general, even a disconnected vac ball won't affect the engine performance much.  How's the idle??

Correct. MAP hose compromised = no pass. Vac system to reservoir hose compromised = pass.

[Bornindesert]

The idle is solid. Steady and smooth, RPMs normal range. Starts easy, no hesitation. I'm going to eliminate the brake power booster and see if that makes a change in the power. I want to do the 96' conversion anyways on this truck so I think that's the next step. The power issue hasn't detracted the kid from putting some miles on the odometer it's just about ready for a diff oil change on the new gears.