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new sierra owner, figuring out what i need to do
https://auszookers.com/forum/viewtopic.php?f=37&t=57922		 Page 1 of 1
Author:  antantant [ Sun Jul 20, 2025 4:23 pm ]
Post subject:  new sierra owner, figuring out what i need to do
Hi, just bought a '90 SJ70 (short wheel base, RHD) that's had a little work done to it. maybe a lift, gearing 175% reduc to 4L, has 31" tires and snorkel (which has been fitted to the carb but no pipe connecting the carb to the snorkel). I flew from central aus to Syd and drove nearly 3000kms to get it home. Noticed a few things though, like the speedo being completely off by about 10% and fuel econ (judging by the distance i traveled measured by google maps and how much i fueled at each servo) was sitting around 13L/100km. I have a job that keeps me out of home for several days at a time so not really much opportunity at the moment to take it to a mechanic, so I wanted to ask here: what would be the heaviest contributing factor to the killing of my fuel econ? I've read some stuff about differential gearing and figure it's probably pretty important for the larger tires it's got, but I'm just not sure if the prev owners have already done that. Zookoffroad seem to only have the 5.37:1's they recommend for 31" wheels, but seeing other posts on the forum make me think that would be overkill. Would the snorkel also not being properly installed also kill the fuel econ?

I had lofty ideas of having a diesel engine installed ideally to bump fuel econ in the right direction too... I want to toy with ideas a bit but just don't know what could possibly be realistic or viable. Finding Australians testimonies on what engines would be good seems like a hurdle in the first place, let alone sourcing the engine and having it installed.

Radiator and aircon upgrades, too? On the drive the base aircon (which, I don't know if it's even aircon or just a toggle box to direct airflow) didn't seem like it was doing amazingly to keep me cool while the sun blared through untinted windows... and living in the desert I wouldn't be too excited for summer. Or am I just being a princess?

Either way, if anyone does reply to this I would be excited for any responses.
Author:  sideways [ Sun Jul 20, 2025 10:43 pm ]
Post subject:  Re: new sierra owner, figuring out what i need to do
Welcome to the forum and good effort on the road trip!

31s are about 18% larger circumference than the stock tyres so your odometer and speedo will be approximately that far out.

Your transfer gears will have dropped your high range gearing as well, just like lower diff gears would in a normal vehicle. What are your revs at 110? If you can pull 5th gear easily enough then the gearing is probably fine.

13l/100 isn't really that bad for a carby Sierra on 31s trying to do 110 down the road. A good one does 10-11 realistically. One of the key problems with sierras is someone has always fiddled with it before and more often than not, they didn't have the knowledge or experience to make it right. We all started somewhere.

In terms of economy problems:
Check base timing and the centrifugal advance actually works. It should be just under 30 degrees at 5500rpm with the vacuum advance disconnected.
Check the vacuum advance diaphragm by sucking on the pipe, it should hold vacuum. These are often busted.
Check for vacuum leaks
Personally I would block the EGR valve and remove the vacuum gear that controls it. It's prone to vacuum leaks and every single one I've ever dealt with has been hooked up wrong.
If you have the original carby they're prone to wear on the bore of the piston that controls the enrichment valve which causes it to be open = worse economy. You can pull it apart and block it off, it really wants a wideband to see what it's doing.
Often the choke is not working correctly. That said if it was stuck shut your economy would be a lot worse.
Fitting a wideband oxygen sensor would tell you what the carby is doing and makes tuning 100x easier/quicker.

If you get a copy the factory workshop manual it has troubleshooting tables for bad fuel economy and all other problems.

There's no reasonably viable diesel swap for a Sierra in Australia. The cars are just so built around a high revving, light weight petrol engine that they need a lot of re engineering to make it work. The Peugeot engines that were factory fitted in Europe were available here but the unique gearbox casing isn't and they don't fit RHD cars. You could potentially go to a under dash (van/truck style) booster/master cylinder to make it fit.

You can make anything work if you're keen enough but there's good reason there's only a handful of diesel swapped sierras in Australia. It's a lot more economical to buy a factory diesel vehicle.

I insulated the firewall and tunnel on my Sierra with closed cell foam and that really cut down the heat in the cab. On hot/sunny days a kiddy shade sock over the door frame is quite nice.

In terms of radiator upgrades I think I've tried everything on the market. The standard plastic tank/aluminium radiators work great, they're only around $120 on ebay. You can upgrade to a 1.6 Vitara/Jimny viscous hub fan. Personally I would fit an overheating alarm (like an engine guard), set it to 95 and never let it get hotter than that. These engines don't tolerate being overheated, they will blow the head gasket.
Author:  HarryHoudini [ Mon Jul 21, 2025 8:55 am ]
Post subject:  Re: new sierra owner, figuring out what i need to do
Welcome.

As stated the Sierra can be thirsty as the motor wears and compression drops,that's without the carby problems that rear their ugly head.

Nice project car that will keep you busy.
Author:  Gwagensteve [ Mon Jul 21, 2025 1:07 pm ]
Post subject:  Re: new sierra owner, figuring out what i need to do
The speedo will be out by 16% but that doesn’t mean the gearing is wrong, the speedo is after the reduction gearing but transfer reduction gearing will correct the gearing for a 31” tyre.
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