Transmission problems... I think
 

I have a '72 Jimmy, 4WD, Turbo 350 Transmission. Because of the issue below, I changed fluids in the trans, trans filter, topped off transfer case (was very low), rear diff, etc hoping it was a dirty trans filter or something in the drive train. No change.

Here's my problem: sometimes in the city but always on the highway, cruising along the truck will be running fine then it starts "lugging" - like its being dragged back or towing 10,000 lbs and I can't go over 60, but then - almost like a shift - runs fine for a few hundred yards then back to "lugging". While 'lugging', if I put the truck in neutral it revs fine & instant response, but as soon as its back in gear, it's back to lugging -- so I don't think its the engine.

Tonight, going about 50 it felt like an improperly set 700R4 feels like when shifting up/down trying to find the right gear -- except this was between "lugging" and running correctly, but not actually shifting gears.

Wish I could explain it better. Any ideas?

Re: Transmission problems... I think
 

I believe if your tranny was going, it would be slipping. You would notice intervals were your RPMs would go up when the tranny slipped, and then back down when it grabbed again and put load on the engine. The lugging sounds like a fuel delivery issue, or intermittent electrical issue with your ignition like you were dropping a cylinder or two and then having them come back on line.

Re: Transmission problems... I think
 

I would tend to agree but the engine performs perfectly in neutral while this is going on. That's what's throwing me. And its not like I'm going up hills so the engine isn't under load, just cruising along. Plus the engine isn't misfiring, sputtering, or runs rough - it just won't accelerate... again, like pulling a 10,000 pound trailer up a hill. Maybe it is an HEI problem?

Re: Transmission problems... I think
 

Many years ago my I was driving my blazer and thought I had Tranny issues. I took it to my Dad who own a service station for 40 years. Even in the era of scan tools in the latter part of his life. He never used one. he drove it around and said Clutch fan. He was right. He told me most people have no idea how much the clutch fan effects transmission shifting etc. About a month a go I am driving my 2003 2500 HD with Allison 5sp auto. I start hearing a noise in the motor or so it seems. It is not running well. Then it goes to where the tranny is not shifting well, High rpms feels like it is slipping. I am praying I make it home and I am thinking what does it cost to rebuild the allison trans. About a mile from home it levels out and goes back to running well. I hear my Dad's voice in my head Its your clutch fan. I check it when I get home Fan is wobble. 50 bucks new clutch fan runs like a new truck. Fairly cheap as well. Just a thought
Good Luck

Re: Transmission problems... I think
 

That's so crazy it just might work!!

Its stories like that when I miss my Dad the most. Owned a service shop all his life and never owned a diagnostic machine. Those "old dogs" new their stuff!

Its been running a little hot lately anyway with a brand new heavy duty radiator, water pump, & thermostat -- so a new clutch fan was already on my list. If I can fix two problems at once - that's a bonus.

Re: Transmission problems... I think
 

If that doesn't work out, I would be looking at the fuel system. There have been several posts on here about people with clogged fuel filters and kinked fuel lines that have literally driven years without their trucks going over 60 mph.

If you have an older quadrajet, check the filter in the carb. These seem to get overlooked.

Re: Transmission problems... I think
 

It could be in the transmission torque converter. It sounds like a locked up stator clutch.

You have to make sure all the engine performance items are in good order

Re: Transmission problems... I think
 

I would agree with the fuel system. My '70 Jimmy has the mechanical fuel pump as well as an inline electric fuel pump. The electric pump is a little noisey and I noticed i couldn't hear it anymore but the Jimmy still ran fine. After the weather got warmer and I was driving it more I started having the same problems as you so I replaced the electric pump and have had no problems since.

Re: Transmission problems... I think
 

I'm traveling the rest of week & weekend so I won't get back to it until next week. I will dig in and post what I find. Seems like fuel is majority opinion where to start. Thanks to all who posted. More to come!

Re: Transmission problems... I think
 

I would check the kick down cable ,maybe unhook it and see if you get the same responce .that might tell you if it's transmission related or otherwise. It's a cheap way to investigate . I suspect your having trouble with bands in the tranny . Did you see any little particle in the tranny filter ?

Re: Transmission problems... I think
 

Could it be a vacuum leak?

Re: Transmission problems... I think
 

For all those that said "Fuel" - give yourself a hand. Finally crawled under to do a deep dive and the fuel pump was "wet". After testing at RPM it was leaking around the seam (not the inlet/outlet). Replaced fuel pump, runs as good as new.

I am a little embarrassed, I should have known better, but I was lazy and assumed because most everything under the hood was new (fuel pump, Edelbrock carb, coil, plugs, wires, etc), I ruled them out without checking. Dumb!

I can't (OK I can) believe this Chinese crap - fuel pump - can't last more than a year. So frustrating!

Thanks everyone!

Re: Transmission problems... I think
 

Change your oil.
If fuel pump was leaking you prob. have fuel in your oil too.

You are spot on - After discovering the pump problem, I pulled the breather and you could smell fuel, so I changed it.
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Re: Transmission problems... I think
 

I love to see a simple fix to what at first might have seemed like a big problem.

Re: Transmission problems... I think
 

I thought I would post a final answer here. After about 10 days, the Jimmy started doing all the same things all over again.

I dropped the fuel tank (which turned out to be surprisingly nice inside), replaced all the rubber fuel/vapor hoses - which on a 72 with all venting routing, it turned out to be a lot of little hoses everywhere. No change.

Replaced the new cheap fuel pump with a higher end fuel pump. Good for one day, then same thing.

Moved to carb... Rebuilt my 'new' Edelbrock 1406 Carb - BINGO! Once I popped it open, i could immediately see what looked like casting sand. I never ran the carb without a new fuel filter - in fact replacing each time I replaced the fuel pump, so I'm blaming Edelbrock on this one! The internal carb filters and jets were clogged with this fine powder. A good cleaning & rebuild -- problem solved. In fact, its never run better.

Of course, looking back I've had several Edelbrocks and this one had never seemed right. Hardest one to tune, keep settings, crappy gas mileage... I trusted the manufacturing process and blamed the engine, vacuum, ignition since I didn't know the engine and replaced the carb & intake as soon as I bought the Jimmy. My mistake... can't take anything for granted. And it didn't help actually having a bad fuel pump to throw me off track.

But now, with 10 days in and running like it never has, this chapter is closed.