No Power in 4th Gear Auto

[Palms06]

Hey everyone,

Long time reader and absolutely love the wealth of knowledge in this forum. Thanks all!
So I have a petrol + gas 1992 80 series with automatic transmission. The truck runs great but once it hits 4th gear at around 80km/h it drops to 1700 rpm and doesn't seem to push past 2000rpm and will keep consistent at this speed until I push down the accelerator until 3rd gear kicks in again. If I can get momentum up in a slight decline then the truck will accelerate and increase speed and revs but as soon as it hits a slight incline the speed backs off straight away. This is really frustrating me and I can't figure it out.

Has anyone encountered this problem? Anyone know what could be causing this?

Cheers,

Luke

 

[Higgy]

Hey everyone,

Long time reader and absolutely love the wealth of knowledge in this forum. Thanks all!
So I have a petrol + gas 1992 80 series with automatic transmission. The truck runs great but once it hits 4th gear at around 80km/h it doesn't seem to push past 2000rpm and will keep consistent at this speed until I push down the accelerator acceleration until 3rd gear kicks in again. If I can get momentum up in a slight decline then the truck will accelerate and increase speed and revs but as soon as it hits a slight incline the speed backs off straight away. This is really frustrating me and I can't figure it out.

Has anyone encountered this problem? Anyone know what could be causing this?

Cheers,

Luke

Hi mate. Does the problem arise irrespective of what fuel your on. Petrol /Gas. have you looked at usual suspects. fuel filter /Tank filter etc.. Is there a flow restrictor valve for the gas etc.. someone will know im sure there are plenty on the forum who run Petrol/LPG Trucks.. Good luck

 

[Palms06]

Hey Higgy,

It happens on both petrol and gas so I have ruled out fuel filter. In 3rd I can get up to 100km/h at about 3000rpm but then as soon as it changes to 4th, it loses momentum and sits on 2000rpm again and slowly drops down to 80km/h unless going on declined flat or down hill as mentioned.
Could it have something to do with my tyres? I have 33inch muddies with 3 inch lift as an FYI.

 

[StarCruiser]

How are your plugs and leads? There was a thread on here from a member with a similar problem. Turned out to be plugs and leads.

 

[Palms06]

Thanks mate - will give them a look over tonight.

 

[Palms06]

UPDATE: The plugs and leads are brand new. I will be checking the air filter tonight to make sure that it is clean and in working condition.

 

[StarCruiser]

Have you only just started driving the truck? Has it done this for a while and slowly got worse or did it suddenly start doing it?

 

[Palms06]

I've had it for a few months now and the issue has always been there. I just assumed it was because of the bigger tyres and weight of the car but did some more research, and realised it should be able to accelerate past 100km/h let alone 80km/h in 4th gear.

Edit - Air filter looks quite new. As the issue happens running on both petrol and gas I can assume its not fuel filter. Perhaps the exhaust?

 

[Palms06]

Could any one here who has a 1992 80 series auto let me know what the revs drop back to when going in to 4th and usual power/top speed? Could be handy in identifying some problems as well.

Cheers

 

[Waldamar]

Could any one here who has a 1992 80 series auto let me know what the revs drop back to when going in to 4th and usual power/top speed? Could be handy in identifying some problems as well.

Cheers

I'm doing my wheel bearings today but as soon as I've done that I'll take it for a run and note RPM etc. Have you got the Overdrive button? on the shift - maybe you're out of o/d and your dash light doesn't work/? but logically that should raise revs so not likely the problem on 2nd thoughts.

Often process of elimination is the way to go - when was trans fluid and filter last changed? If you don't know do it anyway also go for a diagnostic tune up, see what codes are stored....................
Good luck

 

[StarCruiser]

I’m not familiar with the petrol but does it have a catalytic converter? These can sometimes fracture if they are hot and then immersed in water. I would have expected that to cause problems in all gears really.

Process of elimination.

Can you borrow some near standard wheels and tyres? 33s are on the big side so gearing may be a bit tall so it’s out of the power band of the engine.

@Gary820 has a Petrol 80 and possibly largish Tyres maybe he has an idea how it should perform.

 

[Gary820]

Which engine is fitted? Is it a 3f-E (iirc) or 1fzfe?

Mine's a 97 1fzfe and with 35s pulls quite well to 100mph (160kmh) obviously on a private road.


If you drive it manually will it rev right through the rev range? If it does can you turn the over drive off and see what it does in D.
It could be a gearing issue especially if the earlier engine and gearbox. If it'll rev through the other gears without issues the gearing is possible. The earlier one's were a bit under powered.

As suggested the usual fuel air and ignition which seems you've checked.
Can you check the ignition timing?
Distributor cap and rotor arm in good condition? Spark plugs when were they last changed.

Compression test and cylinder leakage test?
Decent lube system for the lpg? As known for wearing valve seats on lpg.

Exhaust restrictions but I don't think Aus spec has catalytic converters?

Any air intake leaks?
Any brakes binding etc.

I find long steep inclines remind me about my gearing, I just drive it like a manual at that point though tbh, peak power is above 4500rpm so it likes to be revved.

 

[Palms06]

Awesome info @Gary820!

It's a 3f-e and am starting to think it might need to be regeared. I can shift through all gears manually from 1/2/3/D and all will shift great.

It has only recently had a compression test, new sparks and leads.

It seems to be dropping low revs every gear change which brings me to gearing.

I'll have it checked out and report back.

 

[nielsc]

You might also check if the cable from the accelerator link to the gearbox is tight enough. This determines shift points on the early models and line pressure in the gearbox depending on accelerator position.

 

[Gary820]

Having a little look on the web the 3fe is known for being a little steady by the look of it, yours maybe a bit too steady though!

As above the gearbox cable is mentioned, could be a simple adjustment. valve clearances also. Hopefully a nice easy fix.

 

[Palms06]

Praying the gearbox cable is the easy fix! Excuse my ignorance but is this the same as the throttle cable?

I've also read that the "D" on the older 440 transmission boxes is the over drive gear. Fair to say running in 3rd gear for my 440 box might be the equivalent of running in 4th on the 442 boxes, hence why the revs drop so much?

 

[nielsc]

4th gear is overdrive, also on the A442f boxes.
The cable goes from the throttle to the gearbox.

 

[Waldamar]

Could any one here who has a 1992 80 series auto let me know what the revs drop back to when going in to 4th and usual power/top speed? Could be handy in identifying some problems as well.

Cheers

Apologies for late reply..........
50 mph = 1600 rpm
60 mph = 2000 rpm
I'm on 285/75 - 16s so I guess 31" or so,

They are steady!

 

[Gary820]

As i was using the 80 today
80 km/h is just below 1800 rpm, it will pull from there without dropping a gear (unless press pedal harder) that's a 1fzfe 97 model running 35's.

Hopefully you're getting somewhere with yours.

 

[joinerman]

My 94 80 when i had it.