Topic: Power and temp (or why is my Jeep a dog when the outside temperature rises)

[95yjester]

Okay, heres the deal '95 2.5l with 4.0l tb w/2.5l sensors, air tube restrictor removed from air box, ford 19lb injectors. Operating temps are normal now engine codes.  When the ambient temperature is cool 40 or below jeep runs great even at highway speed (5 speed with 4.10's & 32's and i can get 65 in fifth gear) last couple days temps have been in the 60's and on my way home on the highway could barely make 55 after jeep got up to normal operating temp guage spikes @ 210 drops to like around 190 at cruising speed.  My question is why such a loss of power with such a minor ambient temp change and is there any thing I can do to change that.

[sharpxmen]

Okay, heres the deal '95 2.5l with 4.0l tb w/2.5l sensors, air tube restrictor removed from air box, ford 19lb injectors. Operating temps are normal now engine codes.  When the ambient temperature is cool 40 or below jeep runs great even at highway speed (5 speed with 4.10's & 32's and i can get 65 in fifth gear) last couple days temps have been in the 60's and on my way home on the highway could barely make 55 after jeep got up to normal operating temp guage spikes  (at)  210 drops to like around 190 at cruising speed.  My question is why such a loss of power with such a minor ambient temp change and is there any thing I can do to change that.

can you maintain 65 in 4th gear?

you lose about 5HP between 40F and 60F

if you want to calculate the difference yourself go here http://www.msdfuelinjection.com/MSDInjectorV1/main.html
use these variables
displacement: 150
max rpm: 5250
inlet air temp: 70 (normal operating conditions), 60 and 40 for your example
#of injectors: 4
estimated VE: 76
air/fuel ratio: 13.5
BSFC (brake specific): 0.47

comes to 120HP at 70F, 121 at 60 and 126 at 40F

[95yjester]

it takes a while to get there but it does get there. 65 is screaming in 4th gear

[sharpxmen]

it takes a while to get there but it does get there. 65 is screaming in 4th gear

what you describe is relatively normal, with 4.10 and 32s the 5th becomes more or less useless. If you want it back you need to regear.

also, make sure you're not getting hot air from under the hood in the intake

[Jeffy]

it takes a while to get there but it does get there. 65 is screaming in 4th gear

What do you consider 'screaming'?

I drove with 4.10's and 32's for 5 years.  BTDT, regear and revel in your new found power.

[neale_rs]

One thing to consider is how much the temp of the air actually going into the intake changes with outside temp.  Has anybody done any testing to see how much difference the outside temp actually makes?  I don't notice much difference myself and it get to over 100 F here every summer.

[sharpxmen]

One thing to consider is how much the temp of the air actually going into the intake changes with outside temp.

if the outside temp is 60 that's what the intake air temp should be at wot (ok, maybe a few deg higher due to the intake being hot). if it's 100 outside, then should be 100 for the air entering the cylinders at wot. If you're drawing air from the engine bay will be higher than the ambient so that's why is better to make sure you get air from outside.

for his example there is a 5HP difference at top power rpm, of course would be less at 3000rpm for example but proportionally (percentage wise) about the same.

not talking when the throttle is not fully open since that creates a vacuum and the temp will be lower than the ambient due to the drop in pressure.

this is all relevant for wot since in between you'll compensate by pressing more or less on the accelerator pedal.

[95yjester]

I understand that it is 4 banger on 4.10s with 32's 5th gear on the thruway is fine. My point is the same drive on the freeway at 35-40 degrees jeep feels as though it still has power @ 60 degrees it has no power I was looking for tips or things that i me be able to check.  Just looking for ideas I have had the jeep for 2 couple years now and am aware of its capacity for shear unadulterated power.

And it is so sexy that .........

[Jeffy]

Are you driving it at the same speed/gear/RPM?  Again, what do you consider 'screaming' to be?  There are many variables that you might not notice like wind as well.

[95yjester]

it's kind of funny because today was nice here high in the afternoon was like 50 on the same stretch of road that I could barely keep 55 mph the other day when it was 60 degrees today i swear i could of hit 70 with no problem if it wasn't for othercars on the road.

[sharpxmen]

it's kind of funny because today was nice here high in the afternoon was like 50 on the same stretch of road that I could barely keep 55 mph the other day when it was 60 degrees today i swear i could of hit 70 with no problem if it wasn't for othercars on the road.

head and tail wind make a big difference on a Jeep since it is shaped like 2 bricks on top of each-other.

[Jeffy]

it's kind of funny because today was nice here high in the afternoon was like 50 on the same stretch of road that I could barely keep 55 mph the other day when it was 60 degrees today i swear i could of hit 70 with no problem if it wasn't for othercars on the road.

What RPM are you running and what do you consider 'screaming'?  This makes a big difference in known if you're in the powerband or not.

[95yjester]

iirc at like 60 mph's on tghe speedo in fifth gear I am 2500 rpm and in fourth gear same speed is like 3000 rpm.  also one thing i noticed today was that in fifth gear at cruising speed the jeep responds better to light throttle than to floor board approach.

[sharpxmen]

iirc at like 60 mph's on tghe speedo in fifth gear I am 2500 rpm and in fourth gear same speed is like 3000 rpm.  also one thing i noticed today was that in fifth gear at cruising speed the jeep responds better to light throttle than to floor board approach.

is 60mph what the speedo is showing or the corrected speed (did you change the speedo gear in the t-case to correct for the 32'' tires)? if it's what the speedo is showing you're in fact traveling at about 68mph

since you have not regeared the 5th is completely out of the power band, your engine does not produce enough power at the new rpm to sustain the speed and overcome the air resistance, even if you floor it, and in fact it will continue to decrease the speed until the consumed power would be equal to the power produced at the corresponding rpm (so forget about gaining speed in 5th).

if you still think that something is wrong with it you can try changing the air filter, o2 sensor and also verify the fuel pressure to make sure it has enough gas. I would also have a look at the ignition but it doesn't sound like there's a problem there (you didn't mention anything).

[Jeffy]

Sounds like he's either lugging it or running WOT in which case it's running lean.

2500-3000 RPM isn't considered screaming.