Throttle won't open all the way

[Deleted]

EDIT: for some clarification, after talking to some people down below this is pretty normal and everything I'm experiencing can probably be traced to the fact that I might have bigger gearing than stock (need to check that) and I'm running 31's on a truck thats 33 years old lol. I made this thread because I had a 94 4cyl wrangler awhile back that had no issues with getting up to speed or maintaining 65-70 that was on 32s so I thought it was odd that my 6cyl was having trouble. But also the wrangler had stock gearing and a much shorter wheel base so I'm sure thats where the difference lies

 

So I discovered after having someone press in my gas pedal all the way, that my throttle body can still open about a good 1/4th (maybe even a half) inch further. Is this normal because I don't feel like it is and it would explain why A.) Ive been averaging about 18-19mpg, and B.) Why my truck is so gosh darn dirt slow. How would I go about adjusting this as ive never dealt with any throttle stuff before.

 

[Pete M]

still rocking the stock 3.07s in those axles?

[DirtyComanche]

Take the intake hose off and see if the throttle is standing wide open at that point.  I believe it should open until the stop, but it might not, it may be full open where it was and is over centering if you push it past that.

 

 

[Eagle]

A wide open throttle plate only allows high RPM operation, it won't boost performance at low RPM. Think about it -- an engine is basically just an air pump. As each piston goes down on the intake stroke, it sucks air in through the throttle body. At lower RPM you don't need it to be wide open and, in fact, making it open wider than necessary for the amount of air the engine can use doesn't improve performance, it hurts it. It also hurts gas mileage.

 

What's your point about 18-19 MPG? Do you think that's better than it should be, or worse?

 

Lastly, as Dirty Comanche already commented, you don't know anything about how far your throttle is opening. You need to remove the intake hose and look at the position of the throttle plate.

[Deleted]

9 hours ago, Pete M said:

still rocking the stock 3.07s in those axles?

i do not believe so, I bought it after the swap and I can't find any information on if it was geared up other than the fact that 1st gear is so low that i normally start in second, I'm able to let off the clutch in first without giving it any gas, and ive climbed some steep hills with no effort at all, and I shift into 5th gear at 45mph. All of that points to low gearing to me but like I said I'm not 100%

[Deleted]

8 hours ago, DirtyComanche said:

Take the intake hose off and see if the throttle is standing wide open at that point.  I believe it should open until the stop, but it might not, it may be full open where it was and is over centering if you push it past that.

 

 

good idea thank you

[Deleted]

5 hours ago, Eagle said:

A wide open throttle plate only allows high RPM operation, it won't boost performance at low RPM. Think about it -- an engine is basically just an air pump. As each piston goes down on the intake stroke, it sucks air in through the throttle body. At lower RPM you don't need it to be wide open and, in fact, making it open wider than necessary for the amount of air the engine can use doesn't improve performance, it hurts it. It also hurts gas mileage.

 

What's your point about 18-19 MPG? Do you think that's better than it should be, or worse?

 

Lastly, as Dirty Comanche already commented, you don't know anything about how far your throttle is opening. You need to remove the intake hose and look at the position of the throttle plate.

i know about the low rpm performance, but on the highway she struggles to maintain even 55-60 so i was thinking if its not opened all the way that could be the culprit. and yes I feel like 18-19mpg is insane for this truck since my old one got 16-17ish, although this one is a manual so maybe thats why. 

[Pete M]

quick and dirty way to tell gear ratio:

put truck in neutral

jack up one rear tire

scratch a reference mark on the rear driveshaft

rotate the tire around exactly twice while counting the revolutions of the driveshaft

that count is your gear ratio (3 and a half times = 3.55, just over 4 times is 4.10, etc)

 

if the axle has a posi, you'll need to jack up both tires and spin one tire just once (meaning the other tire will also spin around once at the same time).   

[Deleted]

25 minutes ago, Pete M said:

quick and dirty way to tell gear ratio:

put truck in neutral

jack up one rear tire

scratch a reference mark on the rear driveshaft

rotate the tire around exactly twice while counting the revolutions of the driveshaft

that count is your gear ratio (3 and a half times = 3.55, just over 4 times is 4.10, etc)

 

if the axle has a posi, you'll need to jack up both tires and spin one tire just once (meaning the other tire will also spin around once at the same time).   

ill have to try that, if i were to guess id say its probably 3.55 but ill definitely find out

[Eagle]

If it's a 5-speed and the axles hasn't been replaced or regeared, it's 3.08. 55 MPH in 5th gear on stock tires is 1591 RPM, which is well below the torque peak of the engine. That's why it struggles. 5th gear is for cruising on level ground or going downhill. It's completely useless anywhere except on Interstate/limited access highways.

 

If your tires are bigger, it's running at even less RPM. Do you have a tachometer?

[gogmorgo]

3 hours ago, Pirate_Staz said:

i know about the low rpm performance, but on the highway she struggles to maintain even 55-60 so i was thinking if its not opened all the way that could be the culprit. and yes I feel like 18-19mpg is insane for this truck since my old one got 16-17ish, although this one is a manual so maybe thats why. 

You shouldn't need your foot to the floor to maintain 60mph, so I doubt not having the throttle absolutely wide open is at fault here. Have you pulled the intake tube off yet to look at what the throttle plate is actually doing?

 

18-19 mpg is what I would consider reasonably normal for an MJ, depending on driving conditions. Some would consider it good, especially with a lift and larger tires. If you're concerned about it or noticed a sudden drop in mpg, regular tune-up items like plugs, wires, cap, & rotor are things to consider. Cleaning up all engine grounds could be helpful as well, but you may be herding cats trying to eek more fuel economy out of it. 

[54bobby]

13 hours ago, gogmorgo said:

You shouldn't need your foot to the floor to maintain 60mph, so I doubt not having the throttle absolutely wide open is at fault here. Have you pulled the intake tube off yet to look at what the throttle plate is actually doing?

 

18-19 mpg is what I would consider reasonably normal for an MJ, depending on driving conditions. Some would consider it good, especially with a lift and larger tires. If you're concerned about it or noticed a sudden drop in mpg, regular tune-up items like plugs, wires, cap, & rotor are things to consider. Cleaning up all engine grounds could be helpful as well, but you may be herding cats trying to eek more fuel economy out of it. 

herding cats, i'm stealing that one. lol

[Pete M]

no need to steal.  that saying has been around for years.    EDS even made a commercial with it.

 

 

[Deleted]

On 6/14/2019 at 6:03 PM, Eagle said:

If it's a 5-speed and the axles hasn't been replaced or regeared, it's 3.08. 55 MPH in 5th gear on stock tires is 1591 RPM, which is well below the torque peak of the engine. That's why it struggles. 5th gear is for cruising on level ground or going downhill. It's completely useless anywhere except on Interstate/limited access highways.

 

If your tires are bigger, it's running at even less RPM. Do you have a tachometer?

Do I have a Tach? yes. does it work? well now youre askin a bit much arent ya. All jokes aside on the highway in 5th gear at 65mph that engine is well above 2k rpm, id say 3500 if i had to guess. I'm running 31s and the axles are out of a 96 cherokee and I'm not sure yet if its been regeared but all signs point to yes.

[Deleted]

On 6/14/2019 at 8:03 PM, gogmorgo said:

You shouldn't need your foot to the floor to maintain 60mph, so I doubt not having the throttle absolutely wide open is at fault here. Have you pulled the intake tube off yet to look at what the throttle plate is actually doing?

 

18-19 mpg is what I would consider reasonably normal for an MJ, depending on driving conditions. Some would consider it good, especially with a lift and larger tires. If you're concerned about it or noticed a sudden drop in mpg, regular tune-up items like plugs, wires, cap, & rotor are things to consider. Cleaning up all engine grounds could be helpful as well, but you may be herding cats trying to eek more fuel economy out of it. 

it was all tuned up just short of being rebuilt last year, only problem is i didnt own it when all that work was done so I'm not 100% sure on if the people who did it, did a good job. Other than this throttle body "issue" i found everything else seems to be in great working order

[schardein]

Ensuring you are actually achieving WOT (wide open throttle) is a tuning step for old school drag racers/hot rodders who are tuning for maximum performance.

 

Most common reason for not achieving wide open throttle is carpet or floor padding/insulation not allowing the accelerator pedal full travel ("pedal to the floor").

 

If you have floor mats under your pedal, or have installed new carpet at some point, or even just have factory carpet-- do some experimenting to determine if you can get WOT by removing the floor mat/padding/carpet.

 

As already noted, look at the throttle blade, not just the linkage.

 

 

[Ωhm]

Backprobe the TPS (KEY ON: pins B & C) at both Closed Throttle (CT) and Wide Open Throttle (WOT) (note voltage readings) using the gas pedal. Take the same readings with gas pedal disconnected and manually move the throttle between the stops. Compare the voltage readings taken. Should be the same.

[Eagle]

7 hours ago, Pirate_Staz said:

Do I have a Tach? yes. does it work? well now youre askin a bit much arent ya. All jokes aside on the highway in 5th gear at 65mph that engine is well above 2k rpm, id say 3500 if i had to guess. I'm running 31s and the axles are out of a 96 cherokee and I'm not sure yet if its been regeared but all signs point to yes.

 

That's impossible.

 

If the axles came from a 4-cylinder, they would be 4.10 gears. On 31-inch tires, with 4.10 gears at 65 MPH in 5th gear the RPMs would be 2292. The deepest gearing you can get for the XJ/MJ with a Dana 30 front axle is 4.88. Even if you had 4.88 gearing, the RPMs at 65 MPH would be only 2728 in 5th gear. And it's highly unlikely that you have 4.88 gears.

 

https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html

 

A related question, since you're running larger than stock tires AND replacement axles -- do you know if your speedometer is accurate? Have you checked it against a GPS? If not, you can download an app for your phone that will use GPS to verify your speed. (Too bad they don't have one to replace the tachometer.)

[JustEmptyEveryPocket]

Eagle,

 

I thought there was a company who starting making 5.13s that would fit in a Dana 30? Anyone know if that is true?

[AZJeff]

The OP talks about having trouble maintaining speed on ground that is not flat and level.  Those of us here in AZ often encounter long mountain grades on the highways, and those are a challenge for high speed driving.

 

For example, my MJ has 3.55 gears and 31 inch tires.  It's also an automatic, but it cannot hold 65mph on long  uphill grades and stay in overdrive.  It keeps wanting to kick down. My solution is to drop it out of OD when climbing grades.

 

My XJ with 3:07's did the same thing, so I wonder if the OP is expecting too much from that driveline arrangement.   Also, don't forget that, when climing moutain grades, you altitude is going up, and that means your air density is going down, so the engine is starving for oxygen to some degree.

[DirtyComanche]

25 minutes ago, JustEmptyEveryPocket said:

Eagle,

 

I thought there was a company who starting making 5.13s that would fit in a Dana 30? Anyone know if that is true?

 

Revolution offers them, don't know who is cutting them or where.

 

Dana didn't offer them for a reason.

 

Regardless, that's only 10%~ lower.

[Eagle]

9 hours ago, AZJeff said:

The OP talks about having trouble maintaining speed on ground that is not flat and level.  Those of us here in AZ often encounter long mountain grades on the highways, and those are a challenge for high speed driving.

 

 

Yeah, I think Arizona is the only place I've encountered those emergency run-off ramps for stopping big rigs that run out of brakes halfway down ...

[DirtyComanche]

1 minute ago, Eagle said:

 

Yeah, I think Arizona is the only place I've encountered those emergency run-off ramps for stopping big rigs that run out of brakes halfway down ...

 

Got 'em all over BC too.

 

Running on flat is absolutely different than in the mountains.  People need to accept that their gear shifting arm is going to get some exercise.

[gogmorgo]

This is true... My 2.5L spends a lot of time climbing hills in 3rd.

[Jeep Driver]

Runaway ramps here in the SE too.