Terrible Mileage in Suburban?

[Brady Veldt]

Let me start off by saying I understand I shouldn't expect 20mpg from a 3/4 ton truck, but 13-14 mpg seems somewhat realistic after some modifications. It's a 1989 Suburban I bought bone stock. TBI 350, 3.73 gears and a TH400 that averaged around 12mpg with a fairly aggressive driving style, but over the past few months I've done some of the basic mods in hopes of a bit better mileage. I went through and rebuilt the throttle body, replaced all the gaskets and such and put a 1" throttle body spacer on it. I also dumped the old air cleaner setup for a 14" x 3" open element. I have 2 1/4" true duals no cats with 2 chamber mufflers and some full length headers. It's mostly tuned up minus new plug wires, new fuel filter and oil filter, pcv valve, oil change, but it might be worth mentioning I have AC Delco platinum plugs, which I heard bad things about on the TBI engines after my purchase. After all of this I've started averaging 10-11 mpg driving like grandpa! I'm getting worse mileage than before while driving lightly in the name of gas mileage. Best I've been able to get was about 14 and that was on the highway at 55, pretty pathetic. My initial thought was that possibly the heated O2 sensor may have been stuck in open loop due to a loose connection or something but after listening to the engine come up to temp and being able to hear the idle drop that seems unlikely, is there a better way for me to tell? Next I ran 2 cans of seafoam through the fuel system, same results. It might be able to use a set of plug wires but I wouldn't expect that to kill my mileage like it has. I've also thought that possibly my injectors may need service, but while the throttle body was apart they seemed in visibly good condition, as did the whole throttle body, and the spray pattern looks fair. Is there something I'm missing? Could my plugs or injectors be the culprit? Has anyone else experienced a drop in mileage from these mods? Sorry for the long post I've just been battling this for the past month and it's really bugging me.

 

[bigblock72]

Put your original air filter back on. The open element air filter is allowing hot air right off of the fan to be drawn into the engine causing a small amount of detonation and the computer is cutting the timing back as a result. The original air filter setup is really pretty good at providing the necessary volume of air and really good at providing quality cool air that is needed for maximum power and mileage, assuming that it is intact. If you want an aftermarket style air filter setup then check ramairbox.com.

The other thing to check is to make sure the base timing is set at 0* or 2* advanced. Base timing is seldom if ever checked and is usually 4 or 5 degrees retarded from timing chain stretch and gear wear.

Is your speedometer accurate? I've seen fuel mileage issues caused simply by inaccurate odometer readings.

 

[diesel4me]

A friend with a TBI 454 in a crew cab dually improved his mpg from 8-10 to 14+ by replacing the thermostat in it with a new 195 degree one from the dealer...he had numerous other things checked,and replaced a few sensors,to no avail..had no codes or check engine light coming on either..

Evidently the one in it was sticking open enough to delay warm up and the computer stayed in cold mode longer,also it took about 15 minutes to get any good heat in the past winter..engine may have been running at 180,he said the thermostat he removed had no markings on it for temperature or brand name..probably some cheapie china junk one..

 

[yellowK20]

Ditch the power and mileage sucking TH400 , a 700R4 , 4l80e , NV4500, hell even a SM465 would be an improvement over the TH400 mileage wise

 

[Brady Veldt]

Ditch the power and mileage sucking TH400 , a 700R4 , 4l80e , NV4500, hell even a SM465 would be an improvement over the TH400 mileage wise

As much as I would love to have an overdrive transmission, it's not in the budget at the moment as I'm 16 and also starting college this fall. But I was told that the transmission was rebuilt and had less than 1000 miles on it when I got the truck in April, so I don't know how much I would really be spending if I could get some cash for this one. But as of right now that doesn't look like it's really an option.

 

[Brady Veldt]

Put your original air filter back on. The open element air filter is allowing hot air right off of the fan to be drawn into the engine causing a small amount of detonation and the computer is cutting the timing back as a result. The original air filter setup is really pretty good at providing the necessary volume of air and really good at providing quality cool air that is needed for maximum power and mileage, assuming that it is intact. If you want an aftermarket style air filter setup then check ramairbox.com.

The other thing to check is to make sure the base timing is set at 0* or 2* advanced. Base timing is seldom if ever checked and is usually 4 or 5 degrees retarded from timing chain stretch and gear wear.

Is your speedometer accurate? I've seen fuel mileage issues caused simply by inaccurate odometer readings.

I will check the timing tonight and put the original air box back on for a tank and see if that helps at all. Thanks for the reply

 

[dyeager535]

For testing closed/open loop, this procedure is what I found referenced a couple of times:

** Service diagnostic mode is entered the same as Engine Off Self Test. Ground out
Pin A and B on the ALDL connector with the engine running. Rapid flashing (approx
1/2 - 1 sec interval) is open loop. Flashing at a slower rate indicates closed loop.

One post I read said start it cold with A/B jumpered, and watch the check engine light. Should go from open to closed loop once it gets up around 160*. Note that some trucks did not go closed loop at idle and heated O2 won't change that, so you may have to drive it steady speed/low load to see closed loop. Or just increase the RPM a tiny bit at idle. Changing how far the O2 is from the original location will affect performance some, and headers almost certainly leaned the engine out, but it's unlikely these would contribute to a loss of highway economy, since exhaust flow and load will be relatively constant and the ECM is likely (but not guaranteed) to be able to compensate for the lean condition at cruise.