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1994 5.7L Tuning, MPG

rlb

rlb

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A few months ago I picked up a 1994 K2500 Suburban 4x4. Curious what kind of MPG you all are getting.

I got 12.5 MPG on a recent highway round trip of about 250 miles. I usually set the cruise at 65MPH. Was expecting ~15MPG. (Let me know if this is unrealistic)
The truck has new tires (cooper H/T). Axles, Tcase have fresh fluid. Transmission is shifting well and overdrive is working fine.

No check engine light. Recently replaced the O2 sensor with a Bosch ($20, I figured what the heck) and spark plugs, wires, cap/rotor are new. Motor runs well although it seems to idle a touch high. The coolant temp sensor on the manifold looks newer. (Had a head gasket job before I bought it, was replaced then). Even when the truck is warm from a long drive, the exhaust smells a bit like it is running rich. Maybe I am just used to the more modern MPI systems with 2+ spark plugs that run (and smell) really clean.

If 12.5 MPG is all I can expect then I can live with that. I bought this as a 2nd vehicle and not for a cheap commuter. But I would like to make sure the truck is running as well as it can. I think my next job is checking the injectors for good flow and ensuring the ECU is getting a proper reading from the coolant temp sensor and not running it too rich.

Any thoughts? Links or advice on accomplishing the above two tasks would be appreciated. I haven't worked on OBD-1 vehicles much.

burban.jpg
 
12.5 isn't horrible, and about on par with a stock TBI engine. Cam swap, some cylinder head porting, and a GOOD exhaust setup can get you up to around the 15-17 mark. Not many people mess with the swirl port heads, but I actually like them for truck use. Also I would ditch teh BOSCH O2 sensor and get a GM or ACDELCO. And the coolant temp sensor should also be a GM or ACDELCO. I've seen way to many of the WELLS ones be off 20-30 degrees brand new out of the box. it would also be a good idea to find somebody with a scanner to hook up to it and see if the sensor readings look correct. The crude OBD1 system wasn't very good for throwing codes when there was an issue.
 
What axle ratio? Without spending a lot of money your best bet would be to try and use non ethanol gas. Good luck finding it though in non premium. Should gain 1 or maybe 2 mpg.
 
Non ethanol regular is more expensive than non ethanol premium around here (separate stores).

I'd do exhaust (manifold back 3" w/ new Y pipe), make sure you have the 3 wire heated O2 sensor, and maybe try flipping the air cleaner lid (free mod). Tbi chips can help maybe.

Just remember, you have a heavy brick w/ a 180hp motor. Swirl port heads are good for mpg and towing. If you were running wot alot I'd say swap to vortec heads when you need to rebuild the top end.

Source Unknown
 
My tbi ownership was short, 5 months with a 1993 4wd burb bfg radial ta's but idk gearing.

I let a friend talk me into 2 things that helped without throwing big $ at it. First I swapped the coil to a higher output msd and played with plug gap, I ended up going around .020 over stock. Had a little more pep in acceleration and I went from 12 to 13 mpg. The other was he pulled off the tbi and polished up the ports in it, I didn't think it would do much but he swore by it and was right. Way better acceleration gain than the coil/plug change, but mpg only went up another .5 mpg. Around $150 for all that, fuel paid for it real quick. Stay on top of fuel filters for those injectors.

The PO already dropped in short headers and 3" exhaust with x pipe and a magnaflow. Idk how much the others would have helped without that. Afaik everything else was stock.
 
You coudl say GM dropped teh ball on teh TBI engines like they did on the 6.5's. I did a 93 for my uncle and he went from 12 on a good day to 17-18. I did another for a friend who seen simliar results. And unless your after top end HP, KEEP the swirl port heads. They pull good to 4500-4800 RPM's with some minor porting, but pull from 500 RPM's on up to the redline with an almost flat torque curve. The VORTEC's get slightly better MPG, but at the cost of RPM's. VORTEC heads don't start really coming on until 3500-4000 RPM's and most don't want to spin a street engine past 6000-6500 for longevity reasons(doesn't leave much of a power band to work with). The stock camshaft was pretty piss poor, and GM did a horrible job with the swirl port castings in cleaning them up. The casting marks and ridges left from the pouring, really defeat the swirl port heads purpose. All they needed was some minor cleaning of the swirl ramps, and the heads come to life. I need to get back out and tear down the 88 I have and do a tutorial on porting the swirl port heads. I took a pressure washer and spray down the intake runner with the valaves out on a non ported and then a ported head, and the difference is shocking at what some minor clean up can do for the flow pattern of the heads. The exhaust isn't horrible as the stock Y pipe flows pretty good. Put on a set of cheap shorty headers(SUMMIT has some cheap ones that have lasted really well, put a set in 8 years ago and they are still holding up nicely. Just painted them with ceramic high temp paint and bolted them on), ditch the cat(or gut it if you have emissions inspections), and go 3" out the back. 94 should already have a 3 wire O2 sensor unless it's a stick shift, but it is EASY to add one in if you have a 1 wire currently.

And the pressure washer sprayed down the runner could be called the broke mans flow bench. It lets you see how the runners and such flow together without spending the money to put it on a flow bench.
 
I had a 95 4x4 w/ 3.42's and got 22mpg, started with 16-17.

Never touched the heads, got shorties for free or I would have stayed stock, ported the tbi and spaced the injectors, flipped the intake lid, 2.5 pipe into a 3" collector 3" cat and Magnaflow 3". I still had stock tuning and cam but it ran alot better.

Source Unknown
 
It has a 1 wire O2 Sensor. If its an easy swap to a 3 wire, i'm all for it.

Educate me on the purpose of the extra wires. I thought it was a heater so that the O2 sensor readings are accurate soon after a cold start.

I pulled the air filter yesterday to look at the throttle body. Everything seemed to be working fine, but its difficult to see the spray patterns. I took a quick video. If you watch in full screen and 720P, you can kinda see the spray pattern.
http://www.youtube.com/watch?v=AJ60ekVi8vA


Gears are 4.10's. I know that's not helping :)

Cheap headers and a better muffler I can do.
 
The 3 wire has the heater which helps the O2 to keep from fouling and reading lean when it is actually rich. Autos for 94-95 normally had a 3 wire, but not all did. For the 3 wire O2 you have the sensor wire that hooks up to the existing O2 wire, and the other 2 wires are ground and power. For the power you tie it in at the EGR valves power feed, and hook the ground up to a good clean engine block ground. I did this to a few and they always ran and idled so much smoother afterwards. To check the spray pattern, you connect a timing light to the engine and aim it at the injectors. The pusling of the timing light will allow you to see the spray pattern better. And 4.10's are defiantely helpign you in the slightest. I bet it tows great with them.
 
12.5 does not seem to terrible considering the truck it is in. Our 02 K2500 GMT 800 Sub scaled at 7200 with two people aboard and fuel. I love them but there are heavy. You would probably get the same mileage with a 454. That 350 is working to move that bulk. That being said Summit Racing makes shorty headers for around $150 that bolt up to the y pipe. Good 8mm plug wires and Ac Delco plugs are the way to go. The cheapo ignition crap in the parts stores is junk. I like Taylor wires or MSD and have always run Ac Delco plugs. Never had one run right with Bosch and Autolites are horrible in my opinion. A good Msd cap and rotor or a good one with brass terminals. I like the Swirl heads as well. The newer Vortecs seem to dink fuel. Know some guys with 2wd 1500 Vortec 350s that get what you get. Our 2wd TBI 350 gets 18-19 in RCLB. They also some air intake kits that should help. Keep up with the maintenance on the TBI. They need to be cleaned and rebuilt frequently like a carb. The passages will plug up and since they are stupid simple they keep running with horrible fuel mileage. Our TBI smells rich too. Most I have been around seem to smell that way. Once you remove the cat and install an exhaust it gets a little stronger.
 
Rebuild the tbi, which is cheep and easy. New oem plugs, wires, cap, rotor, and fuel filter. Check your timing if you can. Also Thefermenator is right make sure the o2 sensor and temp sensor are gm or acdelco. I had a 1990 gmc 1500 4x4 when i bought the truck I was getting around 13 mpg, then after a good tune up which if done by yourself it is cheap. I was getting around 17 mpg. The little filters on the tbi injectors had some junk on them. The injectors had a better spray pattern also after being rebuilt.
 
The 3 wire O2 pretty well takes care of the rich smelling exhaust at idle. Put one in and the idle will clean up ALOT!
 
Chevypoor said:
Cool. Learned something new. All the tbi's I have been around only have 2 wire O2 sensor.
Click to expand...

They are either 1 or 3 for the TBI. The 1 wire is the most common, but in 94 and 95 the 3 wire was used primrily with an auto trans 4L60E(4L80E could go either way). The 1 wire is just a simple signal wire to the ECM from the element, and it is internally grounded to the housing. The 3 wire adds in 2 wires for the haing element(external ground and power for the heating element). And 4 wire O2's like late model cars have uses a ground through the harness for the O2 element.
 
I know the 3 wire helped my in town MPG alot. Helped w/ running rich on cold starts too. The comes out to a fuel savings no matter how you look at it.

All of the little things add up in the end and most are too expensive.
 
Should have said one wire...mid spoke/ typed.... they all were 1 wire. Maybe time to add a 3 wire to the old girl

Sent from my DROID RAZR using Tapatalk
 
I put in remanufactured injectors and rebuilt the throttle body yesterday. Seems to run better.

I think the 3 wire mod is worthwhile and I'll do that soon.

I still would like to know what type of scanner I need to read the coolant temp. It idles high in neutral when warm. I will put an AC delco sensor in there, just haven't gotten around to it yet.
 
I've always borrowed a TECH1 from the dealer when I have one to look at. I think most any decent OBD1 scanner can read them as the GM is fairly common.
 
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