Problem with 5th after manual swap on R32 GTS4

[TheOrangeSkittle]

Hello, so my automatic trans went out on my r32 gts4, eventually got around to swapping it with a gtr transmission.

I used the original speedo gear from the autobox into the gtr trans. And so far in 1sr,2nd,3rd,4th it works fine, but in 5th the car simply acts like it doesn't have enough torque at all throughout the rpm range and wants to stall.   I assume this is because an rb20det doesn't have enough torque for 5th which is made for an rb26dett. 1st 2nd 3rd are pretty decent but 4th needs to stay in boost or it wants to stall. 

 

It has the og auto ecu + diffs.

 

Was curious what could cause this or a fix as I'm unfamiliar with gearing and ratios.  My ideas so far are

 put in an rb25det from a stagea at some point to make 5th usable.    

Take apart transmission, rebuild and swap 4th and 5th

Swap the diffs for gtr ones? Might make it worse though

 

Any advice appreciated thanks 

 

It's a 1991 

[GTSBoy]

The GTS4 was available with a manual, and 5th wasn't particularly taller in the GTR box than in the 20 box (if different at all, and it's probably the same).

So, it is absolutely not that the 20 doesn't have the torque to pull the gear. It should be able to cruise v. nicely at 70 km/h or more in 5th. Not up any slopes, not with any hope of accelerating if you need it, but certainly cruise.

I know this because I went from R32 auto to R32 with manual DET box (and then to 25 with 25DET box and that is where you really realise how little torque the 20 actually has, if you hadn't beforehand!). I wouldn't often use 5th below 80 because the revs were quite low and there was less vacuum and it tended to use less petrol if you kept the revs up a little more in 4th. But it was fine at 80 and up.

So, it is more likely that there is a weird/severe/unexplained problem in the box. Maybe a knackered bearing in the 5th/reverse shaft area.

[TheOrangeSkittle]

3 hours ago, GTSBoy said:

The GTS4 was available with a manual, and 5th wasn't particularly taller in the GTR box than in the 20 box (if different at all, and it's probably the same).

So, it is absolutely not that the 20 doesn't have the torque to pull the gear. It should be able to cruise v. nicely at 70 km/h or more in 5th. Not up any slopes, not with any hope of accelerating if you need it, but certainly cruise.

I know this because I went from R32 auto to R32 with manual DET box (and then to 25 with 25DET box and that is where you really realise how little torque the 20 actually has, if you hadn't beforehand!). I wouldn't often use 5th below 80 because the revs were quite low and there was less vacuum and it tended to use less petrol if you kept the revs up a little more in 4th. But it was fine at 80 and up.

So, it is more likely that there is a weird/severe/unexplained problem in the box. Maybe a knackered bearing in the 5th/reverse shaft area.

Thank you! It's fine if 5th is maybe just no good, I tried going everywhere from 70mph to 100mph and it acts like the car is about to stall. Didn't wanna leave it in 5th too long though. I know 4th and 5th is abit different in a gtr but I didn't think THAT much to where the car is undriveable In 5th.  Also by cruise I'd be going about 70-80mph on highway. Around 130kph I think?  

 

I don't mind trying to rebuild a transmission myself, the worst part to me is the gtr box coming back out, did that solo. Was a nightmare. 

 

So the gear ratios from a gtr box shouldn't be the problem? Because if I rebuild it I will probably only be able to find gtr rebuild kits and gears. 

 

Thanks again. 

[Duncan]

GTS4 and GTR box is the same thing, so that is not your problem. And like GTSBoy said, the diff ratios are similar (10% shorter in GTS4) and even the power and torque are in the same ballpark. I agree there is a problem with the box, not the rest of the setup, this isn't  80S commodore, it can drive up hills without changing down a gear

[TheOrangeSkittle]

34 minutes ago, Duncan said:

GTS4 and GTR box is the same thing, so that is not your problem. And like GTSBoy said, the diff ratios are similar (10% shorter in GTS4) and even the power and torque are in the same ballpark. I agree there is a problem with the box, not the rest of the setup, this isn't  80S commodore, it can drive up hills without changing down a gear

Well looking at the part numbers for the gears and the box between the 2, I think it was 4th and 5th had different ones. And from what I recall the ratios are abit different but I didn't think enough to cause any issue.   But yeah 4th I can cruise in just fine but it really doesn't like not being in boost so gotta keep it around 3200rpm+. 5th just no matter how fast or slow starts shaking the car violently like it's on the brink of stalling at 90mph, like I didnt expect to really accelerate at all but it was so bad it felt like I was hardcore lugging the engine.  I suppose I will slowly acquire all the parts to do a transmission rebuild then. Thanks for your guys' input.  

[GTSBoy]

Just be aware that this might not be caused by the box. It might be clutch/flywheel balance issues, or it could be that there is actually something wrong with the engine that was being masked by the torque converter. Something like badly out is whack valve timing, or low compression or something equally shitty.

[TheOrangeSkittle]

1 hour ago, GTSBoy said:

Just be aware that this might not be caused by the box. It might be clutch/flywheel balance issues, or it could be that there is actually something wrong with the engine that was being masked by the torque converter. Something like badly out is whack valve timing, or low compression or something equally shitty.

It's literally perfect through every gear and all the rpm ranges EXCEPT 5th. Would any of those things cause that in that specific gear? 

[GTSBoy]

Well, just going by your description, it is the engine that is struggling. The struggle could originate in the engine, meaning that it is weak (weaker than it should be) in all gears and only becomes exposed by the tall gearing of 5th.

[TheOrangeSkittle]

20 hours ago, GTSBoy said:

Well, just going by your description, it is the engine that is struggling. The struggle could originate in the engine, meaning that it is weak (weaker than it should be) in all gears and only becomes exposed by the tall gearing of 5th.

I had suspected this could be a thing too, but it is a relatively low mile engine. Only 80k miles on it. And I can understand the it struggling abit more and needing to be in boost to pull 4th. 5th just outright killing it seems abit much. But I don't understand gearing or ratios enough but I'm pretty sure the rb20det makes half the torque at 4-5k rpm than an rb26dett does. I may rebuild the transmission and swap the engine for rb25det in the future, I wanted to anyways. I can definitely feel the car struggling when it's not in boost to pull anything past 2nd, 3rd under 3k if you try and accelerate it just won't. It'll shudder and crawl upto 3.5k where it's no problem suddenly. 

[r32-25t]

Do the auto gts4 have the same diff ratio as the manuals? 

[TheOrangeSkittle]

15 hours ago, r32-25t said:

Do the auto gts4 have the same diff ratio as the manuals? 

I think so? They have a shorter one than the gtr. 

[GTSBoy]

Should be 4.3

[TheOrangeSkittle]

Hello everyone, sorry for the slow responses. I had a chance to drive it on the highway again and I realized I've missremembered what the actual problem is.

 

Okay so there are a couple things going on, not sure if they're related.

So to start, whenever the car starts it starts up just fine but will die immediately unless you give it a blip of gas. It did this before when it was an auto still, and it only recently developed the problem before doing the swap.

It runs through all the gears smoothly, it doesn't like being below 3.5k rpm 3rd and up though. 

To drive on the highway without 5th I need to cruise in 4th gear at about 70-75mph 4.2k rpms,  but doing this for 20 or 30minutes eventually the exhaust/cel light lights up and I let off the gas and after about 10 seconds it goes away and doesn't come back. At least not for quite awhile. 

5th gear is smooth and there is no issue with it but I can't maintain speed in 5th, if I accelerate to 140kph and throw it in 5th it'll slowly die down and lose speed/rpms, and the engine will begin to lug.  If I try and give it gas at all, alot or a little at like 3 or 4k rpm in 5th the engine just lugs hard and is clearly struggling and slowly dies down till I need to downshift. It cannot pull in 5th no matter what without wanting to stall or slowly losing speed. I could probably get it to maintain speed but I'd have to be going something silly like 90 or 100mph down the highway.  

 

Hopefully that's more clear on what's going on. Apologies for the misunderstanding. 

[GTSBoy]

Something's f**ked. Fuel pump, blocked filter, or something else fuel related. Blocked cat or something exhaust related. It is definitely engine, not gearbag, causing it.

[CowsWithGuns]

Sounds like you have engine issues. Compression and/or leakdown test would be my first call (either gain peace of mind or rip the bandaid off) followed by fuel pressure check, vacuum leak check to get your basics covered. IDK where you're at with tools but I've had great luck with ebay fuel and comp testers, and my ebay leakdown went straight to the bin.

Assuming your starting issues are related to the power issue, I wouldn't expect the exhaust to be a (major) concern as if it was really blocked you'd be flatlining power early and it would be fairly obvious. Nothings impossible though!

 

[TheOrangeSkittle]

1 hour ago, CowsWithGuns said:

Sounds like you have engine issues. Compression and/or leakdown test would be my first call (either gain peace of mind or rip the bandaid off) followed by fuel pressure check, vacuum leak check to get your basics covered. IDK where you're at with tools but I've had great luck with ebay fuel and comp testers, and my ebay leakdown went straight to the bin.

Assuming your starting issues are related to the power issue, I wouldn't expect the exhaust to be a (major) concern as if it was really blocked you'd be flatlining power early and it would be fairly obvious. Nothings impossible though!

 

Yes I was probably going to try and install the new fuel filter I have already and see if that makes any difference, just been putting it off because I'm sure 30+yr old hoses are gunna be fun to get off in an awkward position.  Sometimes it will literally start then idle at below 100 rpm unless u tap the gas for a split second then it jumps up to 1k and is fine.  It's weird it does this in 5th only really, it will pull pretty strongly in 1st and 2nd then 3rd only if you're already past 3.5k rpm and 4th if you're past 4k rpm. Any lower in those and it'll struggle for a second then the boost kicks in and it's fine all the way to the limiter.  I figured the exhaust was just from essentially being at peak boost for extended periods of time.   I never noticed it doing anything like this with the autobox though and nothing else changed besides the transmission. 

 

Thanks guys for your advice. After driving it I was thinking maybe vacuum leak or fuel issue or something. I hope it's nothing like compression, I've been pretty easy on the engine but no clue how the previous owner in Japan treated it.  Maybe these issues were just masked by the torque converter. 

[TheOrangeSkittle]

So an update for anyone curious or future people with similar problem, found a couple vacuum lines pretty cracked and leaking, notably the one on the FPR. I replaced them and the cars issue idling at start has vanished and seems to idle much better and return to idle much smoother, previously it'd drop to 600rpm sometimes and settle at 1k eventually. Now it consistently settles at 1k. Drives smoother and pulls harder and smoother with boost on, and seems like it doesnt struggle as much past 3krpm pulling the higher gears though it does still like being around 3.5k-4k rpm.  Will need to get on the highway sometime to test it. 

 

Some of these lines had came off while swapping the trans and I put them back on but due to age they must be working their way back off. 

[joshuaho96]

On 2/12/2024 at 7:22 AM, TheOrangeSkittle said:

So an update for anyone curious or future people with similar problem, found a couple vacuum lines pretty cracked and leaking, notably the one on the FPR. I replaced them and the cars issue idling at start has vanished and seems to idle much better and return to idle much smoother, previously it'd drop to 600rpm sometimes and settle at 1k eventually. Now it consistently settles at 1k. Drives smoother and pulls harder and smoother with boost on, and seems like it doesnt struggle as much past 3krpm pulling the higher gears though it does still like being around 3.5k-4k rpm.  Will need to get on the highway sometime to test it. 

 

Some of these lines had came off while swapping the trans and I put them back on but due to age they must be working their way back off. 

I have a feeling you may have damaged the cat if you still have one. Pull the downpipe/front pipe off and make sure it's not blocked. FPR induced lean-out is not good.

Also, pull the plugs and make sure they're in good shape. Borescope the cylinders if you have one. Compression test the engine.

Edited February 14 by joshuaho96

[TheOrangeSkittle]

26 minutes ago, joshuaho96 said:

I have a feeling you may have damaged the cat if you still have one. Pull the downpipe/front pipe off and make sure it's not blocked. FPR induced lean-out is not good.

Also, pull the plugs and make sure they're in good shape. Borescope the cylinders if you have one. Compression test the engine.

Well I only drove it two or 3 times likes that if I recall correctly. I was getting together a big sort of maintenence package together. With new plugs and coils and filters. So all that stuff I know when it was last changed. And getting a compression tester and such. 

[GTSBoy]

You ran it lean under boost. It could be f**ked.