Gradual power loss

[Roger Slater]

My 1986 Comanche 2.5 is gradually losing power, no where near the performance when it was new.  It now is very sluggish to the point that 5th gear is rarely usable Engine still sounds perfect, no oil burning and excellent oil pressure.   Catalytic was removed when new.  Air cleaner is new.   Total miles now 110,000.  Is this normal Behaviour or is there a specific known cause, valve springs perhaps??

Thanks

Roger

[89 MJ]

I'd start with a compression test.

Has the rear axle ratio been changed?

How does it run when it is cold compared to when it is up to temperature?

[Roger Slater]

Compression test, good idea.

Axle ratios are original.

Starts and runs smoothly, hot of cold. Idles at 750 RPM.   It simply could not pull the skin off a race pudding.

Thanks for suggestion.

[hgeranium]

Fuel pressure is ok?

[pizzaman09]

What's the spark plug condition?

[Roger Slater]

4 hours ago, hgeranium said:

Fuel pressure is ok?

Dealer checked it at 25 psi last week

[hgeranium]

Just now, Roger Slater said:

Dealer checked it at 25 psi last week

That’s quite low. You’re looking at about 31psi at idle

[Roger Slater]

2 hours ago, pizzaman09 said:

What's the spark plug condition?

Clean and properly gapped but will replace them shortly.

[Roger Slater]

3 minutes ago, hgeranium said:

That’s quite low. You’re looking at about 31psi at idle

Dealer checked Pressure at 2500 RPM, said it was higher at idle but did nor record the number,

[hgeranium]

Just now, Roger Slater said:

Dealer checked Pressure at 2500 RPM, said it was higher at idle but did nor record the number,

It could be worth checking out the fuel pump or fuel pressure regulator in that case

[eaglescout526]

12 minutes ago, hgeranium said:

That’s quite low. You’re looking at about 31psi at idle

Hold on there. That’s for 4.0Ls. 2.5Ls need to have 15psi at the throttle body.  Any higher and it would run rich and bypass the regulator. Or tear through the regulator. 
 

 

I had a failing fuel pump do something similar. I’d drive around just fine for a bit, but until everything warmed up is when issues would start. Shutting off and repriming the pump helped but it was an issue I didn’t want to constantly fight with. 

[hgeranium]

Just now, eaglescout526 said:

Hold on there. That’s for 4.0Ls. 2.5Ls need to have 15psi at the throttle body.  Any higher and it would run rich and bypass the regulator. Or tear through the regulator. 
 

 

I had a failing fuel pump do something similar. I’d drive around just fine for a bit, but until everything warmed up is when issues would start. Shutting off and repriming the pump helped but it was an issue I didn’t want to constantly fight with. 

Ahh very good catch. I totally forgot it was a 2.5. My bad!

[75sv1]

Does it have an O2 sensor? Also, I'd check the cap and rotor. Get a brass terminal one if you can. 

[75sv1]

Also, check the wobble on the distributor shaft. You should not feel 'any'. I think the specs are .001.

[Roger Slater]

Thank you gentlemen for your most appreciated suggestions. I think I have spotted a big part of the problem.

Some time back a dealer fitted a new set of tyres. He assumed the truck had the six cylinder engine. Hence he fitted  215/75/15 instead of the OE 195/75/15

These larger tyres drop the Revs per mile from 790 to 730. This in effect causes higher gearing. If the engine had more power the replacement tyres would work very well but the Gutless nature of the 2.5 engine simply can't cope.

 

Roger

[eaglescout526]

What trans is behind your 2.5?

[Roger Slater]

Five speed manual.

[eaglescout526]

Should have a 4.10 ratio out back. Then again. It is 86 so it could be 3.73. But shouldn’t be that bad on power. Both my MJ and XJ are the same engine and transmission, MJ is 4.10 and XJ is 3.73 but the MJ definitely is faster. But I know the 2.5 isn’t that gutless. Better than GMs iron duke and VWs 2.2L that AMC used in the late 70’s early 80’s.