60 MPH max speed?

[Salvagedcircuit]

I've been slowly refurbishing my Comanche back to stock since initial purchase in 2022. The engine sounds excellent and the idle is stable around 650-800rpm. It has been decently reliable so far and I've been using it to run errands around town with no sweat. I have ironed out most of the problems with the vehicle but one remains: I can't accelerate above 60mph on flat ground.
No matter what I do, I have to put my foot to the floor and hold it to approach 60mph. I'm guessing this is not normal? I know these are not speed demons, but I have to keep flooring it to maintain 60mph. I don't know why. There are no weird engine sounds, there are no strange transmission noises. The vehicle does not struggle or hesitate to accelerate from 0-60mph, it just plateaus at 60mph almost like there's a software limiter, but obviously there isn't.

 

Where do I start troubleshooting this? Is my AW4 transmission not shifting into 3rd / 4th? Is is possible that my TPS needs adjustment?

 

Condition:

-IAC cleaned

-throttle body thoroughly cleaned

-new throttle body gasket

-threaded fitting to throttle body for new hose to map sensor. no leaks.

 

[rokinn]

My first thought is catalytic converter.  I'd run a cat cleaner through it to see if it makes any difference at all.

[fiatslug87]

What rpm are you pulling at 60mph?

What gear ratio do you have?

What size tires are you running?

[Salvagedcircuit]

1 hour ago, fiatslug87 said:

What rpm are you pulling at 60mph?

What gear ratio do you have?

What size tires are you running?

1) I have to check this

2) 3.55

3) 235/75R15

[Salvagedcircuit]

1 hour ago, rokinn said:

My first thought is catalytic converter.  I'd run a cat cleaner through it to see if it makes any difference at all.

I can check my cat with a usb endoscope camera. Thanks.

[cruiser54]

I've driven mine with the needle pointing straight down, so yo have an issue.

 

You can check for exhaust restriction with a vacuum gauge:  

CRUISER'S MOSTLY RENIX TIPS

VACUUM TEST FOR EXHAUST RESTRICTION

OCTOBER 31, 2015 SALAD 3 COMMENTS

 

Your vacuum gauge should come with an instruction booklet outlining the procedure.

Hook the  gauge up to a vacuum source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

[schardein]

As an old hot rodder, my thoughts go back to the basics.  

Do you have carpet?  Is there thick padding underneath, or added sound damper products?  Two sets of floor mats?

Have an assistant floor the gas pedal (engine off) while you physically inspect that the throttle blade is going wide open.

No strange noises, like gravel rattling in a metal can on acceleration? This would indicate spark knock, timing to advanced.

Vacuum reading at idle?  Low vacuum could be low compression, a plain old worn out engine.  It could also be a vacuum leak, there is the vacuum reservoir behind the pass side of the front bumper (football shaped plastic), it is easy to forget to plug that vacuum line back in.

Factory air cleaner assembly?  Is the air filter new(ish)?  I've opened air cleaner boxes to find them packed with rats nest, shredded insulation.

Once past the basics, I'm not a Renix system expert, having owned only one (89 XJ Wag) back in the late 90s.

[Salvagedcircuit]

4 hours ago, cruiser54 said:

I've driven mine with the needle pointing straight down, so yo have an issue.

 

You can check for exhaust restriction with a vacuum gauge:  

CRUISER'S MOSTLY RENIX TIPS

VACUUM TEST FOR EXHAUST RESTRICTION

OCTOBER 31, 2015 SALAD 3 COMMENTS

 

Your vacuum gauge should come with an instruction booklet outlining the procedure.

Hook the  gauge up to a vacuum source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

I'm going to check this thanks.

 

 

1 hour ago, schardein said:

As an old hot rodder, my thoughts go back to the basics.  

Do you have carpet?  Is there thick padding underneath, or added sound damper products?  Two sets of floor mats?

Have an assistant floor the gas pedal (engine off) while you physically inspect that the throttle blade is going wide open.

No strange noises, like gravel rattling in a metal can on acceleration? This would indicate spark knock, timing to advanced.

Vacuum reading at idle?  Low vacuum could be low compression, a plain old worn out engine.  It could also be a vacuum leak, there is the vacuum reservoir behind the pass side of the front bumper (football shaped plastic), it is easy to forget to plug that vacuum line back in.

Factory air cleaner assembly?  Is the air filter new(ish)?  I've opened air cleaner boxes to find them packed with rats nest, shredded insulation.

Once past the basics, I'm not a Renix system expert, having owned only one (89 XJ Wag) back in the late 90s.

1. I added 3/4in neoprene rubber below the new vinyl floor. no floor mats. When the pedal is fully depressed, there is still a gap between the pedal and the flooring, so I don't think the flooring is to blame.
2. Great idea, I'm going to do this
3. I will check this, but I don't remember hearing any strange noises
4. I'm going to grab my vacuum tester and check this, thanks.
5. I relocated my canister under my expansion tank, but the rubber lines to it are very old, I should probably replace them, as they are the last lines that are stock.
6. factory air box, air filter has 50 miles on it. I have not observed any oil stains on it.

Thanks tremendously for the input guys!