New suburban, worse mileage?

[pythonguy]

Hey everybody,

Long time reader, first time poster. I had an 85 diesel suburban that snapped a crank last year, and I sent it to the wrecking yard. It was a 3/4 ton 4wd 6.2 diesel with a 4 speed tranny. I loved that truck, so it's no surprise that I replaced it with a much nicer looking (inside and out) 89 burb.

The new burb is also a 3/4 ton 4wd 6.2, but with a 3 speed tranny and a gearvendor overdrive. My trouble is, even with the GV engaged on a long highway drive, the new burb gets a lot worse mileage than the old one! Old burb got 18-20mpg and this one gets 11-13. The GV definitely works and works well, doesn't slip or disengage on me.

So here's what I suspect, I think it's a combination of factors, and I wanted to run it by you folks to see if I'm thinking this out right or if I really have a problem:

1) axle gear ratio. The new burb has a lot more "pep", when i hit the pedal. However before it shifts from first to second, the engine really revs, higher than I recall the 4 speed ever revving. That makes me think the gear ratio on this truck is higher than the old one.

2) no lock up torque converter. On the 4 speed, the TC almost felt like a whole new gear when it locked up. This tranny obviously doesn't have one of those...

3) higher shift points. The truck doesn't shift into 3rd until about 37-40 mph. This seems really high to me, and when I cruise around local street at 30pm the engine is revved pretty high unless I kick in the GV and go to 2.5. I think maybe the last owner had the shift points raised so he could do the "gear splitting" which is frankly speaking totally annoying and not smooth at all considering to go from 2.5 to 3.5 you have to go from 2.5 back to 2, then the tranny shifts to 3, then you can shift to 3.5. I'd rather it shift earlier into all gears and I can use the GV just as a 4th gear.

I don't tow anything and don't really need a high axle gear ratio. I feel like switching as low as I can go would be acceptable for me. Are the axles gears easy to swap?

Thanks!

[Edahall]

Any lift or oversize tires on your 89?

You're losing fuel economy from 2 major things; the lack of a lock up torque converter and heads with larger precombustion chambers. The 89 heads most likely have larger precombustion chambers than the 85 heads.

There are several options for the transmission. You could install an automatic transmission such as a 700R4 or 4L80E with a lock up torque converter. This would also give you double overdrive since you've already got a GV. If your Suburban has 4:11's this would work out good. If its got 3.73's, the engine will lug in double overdrive. Another option would be to find a torque converter with a very low stall rpm for your TH400. A torque converter with a very low stall rpm would have less slippage than your current converter.

For the heads, you could have smaller precombustion chambers installed on your existing heads. This will help fuel economy but at the expense of some power. For best fuel economy, use some precombustion chambers like what they used in the 130hp 1982 6.2 diesel. Another option would be to find a set of heads from an older 6.2 diesel.

FYI, change the oil in your GV every time you change your engine oil. It uses slightly less than a quart of oil so it's not that big of a deal. It's expensive to rebuild so definitely keep it maintained.

[john8662]

I don't think it's the precups in the heads.

I think it's gear ratio and the TH400 transmission.

Although 11-13MPG sounds really low even for a TH400 truck.

[Edahall]

I don't think it's the precups in the heads.

I think it's gear ratio and the TH400 transmission.

Although 11-13MPG sounds really low even for a TH400 truck.

Interesting.... I had always thought one of the reasons for the better than normal fuel economy on my 1982 Suburban was due to the small precombustion chambers. If you're telling me otherwise, I might upgrade to large procombustion chambers and enjoy more power in the future.

[john8662]

Precups do make a difference in fuel economy. I just don't think that's all of the differences in this scenario of why the fuel economy is so much less.

All of the 6.2L precups are small, in comparison to the Turbo "T" cups and Diamond cups found on 6.5L Applications.

I wouldn't think you'd likely find much of a MPG decrease from running the larger of the available 6.2L precups that fit the early (pre -567) heads.

With that said.

The later 567 heads (1991 production and later) have a different injector angle. This angle does several things. The most notaceable is the increased room for a Turbo application later, but there are others. Better staring characteristics and cleaner burning. But, in having a few of them, they don't mileage as well. I had a 1991 R1500 2WD Suburban geared 3.42 with 235/75 rubber and the best it would do was 20-21, this is the 2nd 1991 6.2L suburban I ever had, the other one was around 19-20 best on the highway at 70mph.

Then take my older pre-567 headed 6.2 in my 86 and it did 24-25MPG at 70 with 3.73 gears and 31" tall rubber. Granted it's a truck and is likely to weigh less than a suburban. Still makes me think the early heads to MPG better, especially ones with the larger exhaust valves (like early 82) and actually my 86 had (LD 6.2 84-86).

But in the scenario above, it's gearing, no locking converter, and could be too much fueling due to a rebuild or improperly calibrated injection pump?

Does it smoke?

[Edahall]

According to Wikipedia, it looks like power output of 130 hp for the 6.2 stayed virtually the same during its production.

Another possibility is maybe 6.5 heads with the large were installed on his 6.2.

Engines:
Engine Years Power Torque Notes
4.3 L V6 1988–92 160 hp (120 kW) 235 lb·ft (319 N·m)
4.3 L V6 1993–95 165 hp (123 kW) 235 lb·ft (319 N·m)
4.3 L V6 1996–98 200 hp (150 kW) 260 lb·ft (350 N·m)
5.0 L V8 1988–95 170 hp (130 kW) 255 lb·ft (346 N·m)
5.0 L V8 1996–99 230 hp (170 kW) 0 lb·ft (0 N·m)
5.7 L V8 1988–95 210 hp (160 kW) 300 lb·ft (410 N·m)
5.7 L V8 1996 250 hp (190 kW) 330 lb·ft (450 N·m)
5.7 L V8 1997–99 255 hp (190 kW) 330 lb·ft (450 N·m)
6.2 L V8 Diesel 1991 130 hp (97 kW) 240 lb·ft (330 N·m)
6.2 L V8 Diesel 1992–93 143 hp (107 kW) 257 lb·ft (348 N·m)
6.5 L V8 Diesel 1994–99 180 hp (130 kW) 360 lb·ft (490 N·m)
7.4 L V8 1990 230 hp (170 kW) 385 lb·ft (522 N·m) 454 SS truck

[Edahall]

According to Wikipedia, it looks like power output of 130 hp for the 6.2 stayed virtually the same during its production.

Another possibility is maybe 6.5 heads with the large precombustion chambers were installed on his 6.2. Something is causing this newer 6.2 to have a lot more "pep".

Have you found out what rear end you have? It's possible your 89 has the optional 4.10's.

Engines:
Engine Years Power Torque Notes
4.3 L V6 1988–92 160 hp (120 kW) 235 lb·ft (319 N·m)
4.3 L V6 1993–95 165 hp (123 kW) 235 lb·ft (319 N·m)
4.3 L V6 1996–98 200 hp (150 kW) 260 lb·ft (350 N·m)
5.0 L V8 1988–95 170 hp (130 kW) 255 lb·ft (346 N·m)
5.0 L V8 1996–99 230 hp (170 kW) 0 lb·ft (0 N·m)
5.7 L V8 1988–95 210 hp (160 kW) 300 lb·ft (410 N·m)
5.7 L V8 1996 250 hp (190 kW) 330 lb·ft (450 N·m)
5.7 L V8 1997–99 255 hp (190 kW) 330 lb·ft (450 N·m)
6.2 L V8 Diesel 1991 130 hp (97 kW) 240 lb·ft (330 N·m)
6.2 L V8 Diesel 1992–93 143 hp (107 kW) 257 lb·ft (348 N·m)
6.5 L V8 Diesel 1994–99 180 hp (130 kW) 360 lb·ft (490 N·m)
7.4 L V8 1990 230 hp (170 kW) 385 lb·ft (522 N·m) 454 SS truck

[pythonguy]

Well thanks so much for all your replies. I looked up the RPO code from the glove box sticker and it says I have 3.73 real axle ratio, which seems fairly standard, right?

As for precups, i guess I could be running with some different heads, the guy who owned this truck before clearly was a wacky gearhead and made a lot of little modifications, some of them kinda weird. He added manual cold start switch, which is a little annoying to have to switch on and off, and a manual glow-plug relay cut-off switch. Not a manual glowplug override, but a cut-off. It seems useless to me, I'd rather be able to glow more, not less!

That being said the guy did baby this thing, the engine is clean, it came with a full documented history of oil and fuel filter changes, and the GV fluid is super clean (I did change it anyway).

So, I wouldn't be totally surprised if I had 6.5 heads on this block or something like that, but I can't confirm.

The suggestion to get a lower stall TC, will that really help? The lowest I see on sites like jeggs and summit is 2000rpm, is that much lower than stock? It seems like a little less slippage would be nice, but I don't know what the downside is, less pep off the line?

I'm beginning to think, that this is just one of the big diffs between the th400 and the 700r4. I figured with the GV the difference wouldn't be that huge, but, it seems to be the case. I can't think of anything else, the air and fuel filters are clean, it's not leaking fuel out onto the road, and it doesn't blow any black smoke, just a little haze when I floor it.

[Edahall]

Sounds like we might be on to something. Take a picture of your heads so we can identify if it's some heads off a 6.5L.

3.73 axle ratio was the standard for your Suburban. 4.10's could be ordered as an option. 3.73's would cause the engine to lug with double overdrive (GV behind a 700R4 or 4L80E). If it were me, I would install a SM465 manual. They're inexpensive and very strong.

Below is a link for a 1000 rpm stall torque converter.
http://www.summitracing.com/parts/HUP-GM5XFM-1/
It definitely would help with fuel economy but the question is how much. This is a popular upgrade for 1st generation Dodge Cummins owners who had trucks with no lock up torque converters. It's one of the best upgrades you can do for these trucks from what I've heard. A diesel engine has much less need for torque multiplication like a gas motor because it has much more low rpm torque. I have yet to hear from someone who has done this on a 6.2. But if you do go this route, please be sure to let us know how it works out.

[john8662]

1500 is the stock stall.

If the trans was rebuilt at some point, maybe it has a different stall.

I have the same truck you do that I may be putting back on the road soon.

1989 Suburban 4x4 3.73 3/4 ton, had a n/a 6.5 in it that was blown up, I'm installing a 1987 6.2 C series engine (what I had on hand). It's also got the TH400, but no gear vendors. Be interesting to see what it mileages. I'm seriously considering a TH700 conversion. I will be replacing the convertor because I think the one in it's bulged, cause something took out the thrust main bearing in the 6.5 engine that was in it. I may opt for lower stall while I'm at it.

I've tried as low as 1200 stall in my drag racing truck, not much difference in performance really. I saved it for a driver TH700 truck later on.

[convert2diesel]

Have you checked the speedo with a GPS? Don't know how the GV is hooked up, but by 89 they were using the electronic speedo sender on these trucks (at least my 89 GMC pickup did). Still had to use the right gears in the tailstock, but electronic none the less.

Could be the PO didn't get it right.

The 89 GMC had the 6.2, 400 auto and 3:73 gears running stock rubber (245x16) and consistantly got 23 mpg on the highway without an overdrive (loaded or empty). With the GV you should be getting close to 25.

Just a thought.

Bill

[pythonguy]

The speedo is pretty much on the money.

25mpg, sheesh, I wish! That's almost double what I'm getting now! I sure hope I can fix this. :)

So here's my logic, it's consuming too much fuel, it must be going somewhere, it's not leaking, there are no drips in my driveway and it I've had a diesel leak before, it's apparent from the truck smelling like a refinery when hot. It doesn't seem to be coming out the tailpipe because you'd think I'd be blowing black smoke everywhere... so I'm stumped. Stuck brake caliper? Wouldn't it be overheating and smoking? Just had the diff fluids changed and and alignment and the wheel bearings inspected and they were all good.

I'm going to try for another data point, I'm driving from Portland to Bend this weekend, I'll top it off, do the trip, and top it off to the exact same level again with the same pump at the same station. I'll report back and you guys know what the new number is.

This isn't a huge loss for me, but obviously I'd love to improve the situation, the truck is only for weekend snow and mud trips to the mountain. My daily driver is an 84 Mazda B2200 with a 2.2L S2 N/A diesel. Mazda's short-lived perkins clone. It's got no power, blows hazey smoke everywhere, sounds like an old VW with a hole in the muffler, and gets 40+ mpg :)

-Mike


Have you checked the speedo with a GPS? Don't know how the GV is hooked up, but by 89 they were using the electronic speedo sender on these trucks (at least my 89 GMC pickup did). Still had to use the right gears in the tailstock, but electronic none the less.

Could be the PO didn't get it right.

The 89 GMC had the 6.2, 400 auto and 3:73 gears running stock rubber (245x16) and consistantly got 23 mpg on the highway without an overdrive (loaded or empty). With the GV you should be getting close to 25.

Just a thought.

Bill

[Edahall]

Which do your injectors look like?

http://img.auctiva.com/imgdata/1/0/4/3/6/7/5/webimg/218225758_o.jpg