M998 speed (or lack of it)

[EladB]

Last night I went and looked at a HMMWV that is for sale. I'd never driven one before, so I don't know what's normal and what's not. The last time I drove anything painted flat green it was an M151 or an M880.

Overall the truck was in pretty good shape. It's an ex KS NG M998, manufactured in 1992 and has about 35K miles on the clock. It looks pretty good and started right up, idled and ran fine. I took it out for a short drive on the gravel road. It seemed to drive OK, but I was unable to get it over 40 MPH. On the way back to the guy's house I had it floored, and the speedo still never got above 40.

Is this normal, or a common sign of a specific problem? Transfer case stuck, air intake blocked, etc... I've already decided that this particular vehicle is not for me -- it has an off-road-only title and I'm not in the mood to try to fix that. Just want to know if it's something easily explained.

I did try cycling the transfer case from HL to H to L. When it was stopped and idling it seemed to shift fine, any other condition it wouldn't. I'm not a 4WD kind of guy, couldn't remember what I had read about a function check, and by then I'd found out about the title so I didn't do any in depth investigation.

 

[Action]

This is why folks dont drive my vehicles. Was it revving really fast and lpud? As in high rpm?

 

[EladB]

Didn't seem to be, no. No tach, of course, but RPMs seemed to be in a very reasonable range. I'm not experienced with Humvees, but I'm not a complete idiot.

 

[EladB]

I think I might have figured it out. I'd never seen one of these before:



but the one in this HMMWV was pegged at the top. I suppose a restricted air filter would cause the issue... I have ever driven anything with a clogged air filter (why wouldya?) but I suppose it might just limit speed like that. I didn't notice any obvious black smoke or strong exhaust smell. I don't know if the fuel injection uses throttle position or MAP for metering the fuel (not a diesel guy either). Would that do it?

 

[big block 88]

It would have been down on power and most likely smoking heavily under throttle.

Not to be a dick but maybe check the operator tm and learn some about 4 wheel drive and diesels. Military vehicles arent for wveryone. They need upkeep and a knowledge of their systems.

 

[EladB]

Gosh, sorry I didn't get all the operator training done prior to taking a five minute test drive of one for sale. Point taken, but maybe I should have been more thorough in my original post. I didn't want to write a novella about it, just ask the question.

I've driven military vehicles before, and am familiar with the concept of operator maintenance and PMCS. I've also driven 4WD vehicles, but it's been a lot of years since I dealt with a manual transfer case -- and then it was an M151 mostly on paved roads, so not a lot of dicking around with it. I've driven a few diesels, but they were generally rental trucks and in decent repair. This particular experience was new, but having never driven an M998 before I don't know if this one's got a problem or if they just naturally can't get out of their own way.

This one isn't mine, not do I know anything about its history, so it got the "Does this thing appear to be reasonably safe to take for a brief test drive" inspection. No obvious defects were noted. Suspension was tight with no play, no leaks, steering felt normal, tires in good shape, engine started and ran fine with no unusual noise or smoke. I'm certainly not an expert but also am not a complete moron on most days, I would probably have noticed if it had been over-revving, billowing black smoke, shaking, grinding, parts falling off, etc. The only unusual thing I noticed was that it was slower than expected (even for a military 4WD) and didn't seem capable of exceeding 40. As it was a vehicle I didn't own on a gravel road and I didn't really want to take it out on the highway, I didn't push it too hard.

Just asking whether it would normally take a lot of coaxing to get an M998 up over 40 on a half mile or so of road, or not. This one may have problems, but they're not my problems since I'm not buying it.

 

[Lionel]

For a 3 speed you should have been able to get it up to 55 or 60 without screaming. Maybe its not shifting into 3rd. Sometimes its hard to feel the shift points. Also, the gauges are notoriously inaccurate. Not sure about restricted airflow behavior.

 

[98G]

I'm reasonably close you, about 45 minutes from Beatrice NE.

I have a good M998 with an on road title and current plates. Everything works like it should. You're welcome to come over middle of next week or possibly the weekend and drive it. This'll give you a basis for comparison for future test drives.

Mine (6.2 3spd) is happy going down the road at 50-55mph.

(This is not a for sale ad)

I may have new wheels and tires on my M925 by then also, and it'll need driving too.....

 

[EladB]

For a 3 speed you should have been able to get it up to 55 or 60 without screaming. Maybe its not shifting into 3rd. Sometimes its hard to feel the shift points. Also, the gauges are notoriously inaccurate. Not sure about restricted airflow behavior.

It's possible; in fact, it was one of the things I wondered about. The surface was rough enough and the vehicle unfamiliar, so the shift points weren't obvious. I'd guesstimate the RPMs were somewhere around 3K or a little higher. Might even have been a combination of factors. I was surprised by two things: first, the driver's position was much less roomy (leg room) than I expected. Second, I wasn't expecting sports car like performance, but I think a golf cart would have outrun this thing.

I'm reasonably close you, about 45 minutes from Beatrice NE.
8<
Mine (6.2 3spd) is happy going down the road at 50-55mph.

I appreciate the offer, thanks. If there's an airport near you I might drop down there one of these days if the weather's nice and you're around on a weekend. Swap you rides.

 

[JoeJrTheBarber]

Most M998's with the 3 speed can cruise around 50-55mph, and pushed to 65-70mph.
I would guess the truck you drove was in need of a simple go over / tune up but I'm no Humvee mechanic.
Also get used to the attitude on this forum, some members would rather bash then help...

 

[gimpyrobb]

Not sure about hmmwvs but are you sure the speedo was right? My deuce and 5ton never are.

 

[EladB]

Most M998's with the 3 speed can cruise around 50-55mph, and pushed to 65-70mph.

I would guess the truck you drove was in need of a simple go over / tune up but I'm no Humvee mechanic.
Also get used to the attitude on this forum, some members would rather bash then help...

No big deal. Check out an aviation forum some time. And I'm sure that there is no shortage of really, really dumb people asking really, really dumb questions here. I can certainly understand an assumption in that direction.

If I were looking at, say, a 5-ton, or something else completely out of my range of experience, I'd get someone who knows what they're doing to ride along. A HMMWV is not exotic enough that a person of average experience after 40-odd years of driving couldn't reasonably expect to be able to take it for a short test drive without breaking things.

In the end -- all I really needed to know was either, "This is normal" or "This vehicle has a problem of some sort". Don't really even care much what that problem is, since it's not my problem. I'm not expecting it to drive like the F150, just getting a feel for whether I should even keep looking. It's not like I can find a few in town to check out, so the next test drive will be a pretty long trip. This one was ~150 miles round trip, well worth the time and miles even if the vehicle was DQ'd.

 

[EladB]

Not sure about hmmwvs but are you sure the speedo was right? My deuce and 5ton never are.

I'd not be surprised by a small error, but I'm pretty sure it was at leeast close. No way was I going much over 40, if that.

 

[98G]

[/FONT I'm not expecting it to drive like the F150.

My girlfriends daily driver is a 2016 fourdoor wrangler, lifted on 35's. I'm actually surprised at how close the hmmwv feels to the jeep, driving down the road at 55mph.

Given, the jeep has twice the hp, and a much higher top speed. But at 50-55 they don't feel *that* different.

 

[Action]

It's possible; in fact, it was one of the things I wondered about. The surface was rough enough and the vehicle unfamiliar, so the shift points weren't obvious. I'd guesstimate the RPMs were somewhere around 3K or a little higher. Might even have been a combination of factors. I was surprised by two things: first, the driver's position was much less roomy (leg room) than I expected. Second, I wasn't expecting sports car like performance, but I think a golf cart would have outrun this thing.

I appreciate the offer, thanks. If there's an airport near you I might drop down there one of these days if the weather's nice and you're around on a weekend. Swap you rides.
View attachment 724885

The driver seat is somewhat adjustable. There should be a small data plate with a picture near it to show how to adjust it.

 

[Skrilex]

I don’t have much experience with these but it sounds like it’s not getting fuel. The air filter thing is suspect but as was pointed out if it was fueling enough without air it would’ve been smoking.

Reminds me me of a problem on my newly acquired old dodge Cummins. It had a stuck fuel pin in the afr that was preventing fueling. Still ran and drove fine, would do 65 but was just very slow. The pin was gummed up with varnish.
Now I’m not sure if hmmwv has something like the afr on it because no turbo, but just relating to similar symptoms.

 

[LouWon]

Mine is a 1989 M998
Max speed is 71.5 MPH, you can download an app on your IPhone for vehicle speed
Highway speed is 60MPH and the tach is at 3K RPM

 

[98G]

Max speed is 71.5 MPH,

Not for long....

 

[LouWon]

Not for long....

Yep. That was top speed on a flat road. 60 is the best speed for it, might go 65 for passing. But that’s it.


Sent from my iPhone using Tapatalk

 

[98G]

71.5mph = impending installation of the coveted 6.2 block interior inspection port.