why is my '91 a slug?

[4x4rob]

I just bought a '91 XLT Explorer 4x4. The engine has good oil pressure, a clean air filter, doesn't smoke, miss, or knock, but it's slug. Here are the specs: '91 XLT, 4.0L, 5 spd, 4x4, 3.73 gears, 30x9.50x15 tires, and completely stock. If I dump the clutch it won't even spin a tire on the pavement. I had a '93 EB 4x4 with an automatic and something like 3.27 or 3.42 gears before but I don't remember it being a dog like this one.

And it's not lighting the check engine light like there is a problem or code either.

any idea is appreciated.

Rob

 

[IZwack]

If the vehicle hasnt had a tune-up in a while - that'd be the first step (spark plugs, spark plug wires, oil, oil filter, air filter, etc..) . Also get the engine "flushed" (or whatever they call it at the dealership) before you do the tube-up. Its about $100 - kinda expensive but when it was done to my 98 Explorer at ~90k miles, it made a quite a difference in acceleration and increased my fuel efficiency by about 15% (on a drive from DC to Rochester, NY).

 

[savagefan]

Hi miler? how's the chain?, how's the exhaust? throw a vac guage on it and see whazzup.

 

[old mechanic]

I just bought a '91 XLT Explorer 4x4. The engine has good oil pressure, a clean air filter, doesn't smoke, miss, or knock, but it's slug. Here are the specs: '91 XLT, 4.0L, 5 spd, 4x4, 3.73 gears, 30x9.50x15 tires, and completely stock. If I dump the clutch it won't even spin a tire on the pavement. I had a '93 EB 4x4 with an automatic and something like 3.27 or 3.42 gears before but I don't remember it being a dog like this one.

And it's not lighting the check engine light like there is a problem or code either.

any idea is appreciated.

Rob

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4,000 lb truck, wide tires and the highest gears. Not a good combo to spin any tires.

 

[4x4rob]

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4,000 lb truck, wide tires and the highest gears. Not a good combo to spin any tires.

3.73 gears are considered "low" gears, especially compared to 3.27s or 3.42s. They are numericaly higher but the ratio is lower.

Maybe it's just me because I'm comparing it to my '90 Jeep Cherokee, 4x4, 4.0, 5 spd, 3.55 gears, and 33" BFG Mudterrains. I realize it's 1000 lbs. lighter that the Explorer but still...

Vaccum is 19 lbs. at 900 rpm, ac off, 5 spd transmission. How does this compare?

Oh yeah, I just changed plugs and wires which made only minimal difference. In the process of this, I checked compression. All cylinders had close to 150 lbs. each.

thanks.

 

[old mechanic]

3.73 gears are considered "low" gears, especially compared to 3.27s or 3.42s. They are numericaly higher but the ratio is lower.
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I understand ratio's but I don't consider anything LOW until they get into the 4's. Last time I raced I was using 456's.

 

[4x4rob]

3.73 gears are considered "low" gears, especially compared to 3.27s or 3.42s. They are numericaly higher but the ratio is lower.
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I understand ratio's but I don't consider anything LOW until they get into the 4's. Last time I raced I was using 456's.

ok, but they are not "highest" as you described them before. And the last time I raced, I was using 3.90 gears in a Moser 9" in a street legal '93 Z28.

Rob

 

[old mechanic]

ok, but they are not "highest" as you described them before. And the last time I raced, I was using 3.90 gears in a Moser 9" in a street legal '93 Z28.

Rob

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Yup, I should have said high gears compared to any of the 4's.
Whatever gears wind you out right at the end of the quarter mile is what is needed.
AH, the good old racing days when I ran a "FAST" 10.9 130 MPH. LOL!

 

[Rhett]

I just bought a '91 XLT Explorer 4x4. The engine has good oil pressure, a clean air filter, doesn't smoke, miss, or knock, but it's slug. Here are the specs: '91 XLT, 4.0L, 5 spd, 4x4, 3.73 gears, 30x9.50x15 tires, and completely stock. If I dump the clutch it won't even spin a tire on the pavement. I had a '93 EB 4x4 with an automatic and something like 3.27 or 3.42 gears before but I don't remember it being a dog like this one.

And it's not lighting the check engine light like there is a problem or code either.

any idea is appreciated.

Rob

Because you only have 155 horsepower, a 4,000 lb truck. I see you comparing it to a Cherokee. There's zero comparison, as the Chero is much much lighter. Cherokees with that excellent 4.0 I6 are very peppy. The 4.0 OHV in our trucks is built for torque (225 ft lbs at only 155 hp in the case of your 1991) and yet don't expect it to lay big tire scratches.

The '93 EB has 160 horsepower. That 5 hp difference is almost insignificant though. Maybe your 91 has a California Emissions computer program. That will cut your power some more. Just because you are not in California, you still may have a Cali Emissions PCM. Mine did, before I chipped it--and the truck, built in Kentucky, has never been to California.

Basically, there are a lot of variables as to why your 91 is doggy. If you have it perfectly tuned, (Motorcraft plugs and wires, stock gap), and so forth, it STILL won't be fast.

 

[savagefan]

Put that vacuum guage on and measure at different rpms, if it drops as the speed increases, plugged exhaust. Physically check to see if your pipe has been squashed anywhere. How's fuel pump pressure?

 

[4x4rob]

I appreciate all this information.

I disconnected teh exhaust and discovered that someone had gutted the cats. Since I already had it disconnected from the mufflers, i drove it around the block to see if it made a difference in power but it didn't.

Dang, I figured this would be more peppy especially with a 5 spd and 3.73 gears. I order a suspension lift and body lift so I can run 33x15.50x15 Super Swamper, but I don't think the truck will pull them in stock form. Just for kicks I mounted these on my Cherokee tonight. Excpet for rubbing on the fenders when I'm turning, it runs like a champ.

Now I'm having second thoughts about keeping the explorer.

Rob

 

[Brandons]

Sounds like mine, gutted cats, but 3.27. Not very gutless either, tune ups your best bet and a few simple mods will help

 

[Creager]

gutted cats... less back pressure? I'm pretty sure the ford 4.0 is one engine that relys on back pressure for power.

I was running straight headers when i was breaking in my new motor, my built 4.0 struggled with the low-end. I put the cats back on and the low-end power doubled. This thing will tear past a cherokee in first gear.

3.73 gears and a 4.0 5speed should actually be fun to drive. I say try getting some cats on there, see what happends.

 

[unpredictable1]

gutted cats... less back pressure? I'm pretty sure the ford 4.0 is one engine that relys on back pressure for power.

I was running straight headers when i was breaking in my new motor, my built 4.0 struggled with the low-end. I put the cats back on and the low-end power doubled. This thing will tear past a cherokee in first gear.

3.73 gears and a 4.0 5speed should actually be fun to drive. I say try getting some cats on there, see what happends.

With a 5 speed he should definitely be able to compensate better for the lesser back pressure than an auto would. I don't think thats the prob. How is the intake? the MAF? what about the Throttle Body? is it gummed up/dirty and not opening fully?

 

[mikehawk]

My 91 had clogged cats. After the exhaust shop fixed that it had a lot more power. Still not fast, but at least it doesn't take forever to get the thing moving. And clogged or gutted cats will not throw a CEL because the 91's use OBD1.

 

[Brandons]

Thats why i punched mine out

 

[Creager]

With a 5 speed he should definitely be able to compensate better for the lesser back pressure than an auto would.

I see this the other way around. Auto = Torque multiplication.

A 4.0 with less back pressure is a dog, maybe a little quicker in the upper end... actually no, the less pressure affected the upper end negativly too. When my 4.0 was stock it would easily torque my neck back if i hit the gas right.

I put my cats back on and the power went up across the board, even just having the oxygen sensor bungs open and unplugged I had a noticable loss in power.

I'm also driving a sport, i dunno how a 4door is like. 4x4rob, maybe you should save up your cash for a 5.0 conversion?

 

[Rhett]

I disconnected teh exhaust and discovered that someone had gutted the cats.

That is a problem right there. The OHV likes to have back pressure.

Sounds like the truck just needs some TLC (tender loving care). Get some cats on it, make sure the muffler's right, make sure the plugs and wires are right, air filter, fuel filter, fuel pressure regulator, radiator, O2 sensor, stat, fan clutch, computer, vacuum, throttle body (check the throttle cable for stretching), run some Sea Foam through it, and change the oil.

Guarantee you'll have a 100% better running truck after all that.

 

[FROADER]

That is a problem right there. The OHV likes to have back pressure.

It also doesn't like to push 4500lbs. with only 150hp in a 15 yr old engine.

 

[chakaD]

How is the clutch biting? A slipping one will defintily kill your get up and go.