Deteriorating gas mileage

etex211 · August 3, 2004

I've got a '96 Chevy crew cab dually with a vortec 454. I use the truck almost exclusively to pull my race car trailer. I've had the truck for a little over 4 years and I've consistently gotten about 7-7.5 mpg pulling that trailer. All of a sudden (in the last few months), my gas mileage has dropped to about 5.5-6.0 mpg. This is costing me money and I'd like to get it fixed.

Recent tune-up items that I have done include new plugs and wires, new air filter, new fuel filter. The truck has 128,000 miles. The "service engine soon" light has been coming on intermittently. I had the code checked at a local parts store and the code (I can't remember the code number) indicated something in the area of the EGR valve.

Some other things that I have observed: The temp appears to be about 160-170 degrees on the gauge. This seems low. The inside of the tail pipe looks black and sooty, which would seem to indicate a rich condition to me.

My questions: Can the EGR valve cause the deterioration in the gas mileage? I really don't want to have to buy one of those things, but if it will fix my gas mileage problem, I'll buy one this week. Do the water temp and tail pipe symptoms mean anything? Is there anything else that I need to look at?

I found this site after doing a google search, and this is my first visit to the site. It looks like a pretty neat resource. Thanks in advance to anyone that can help me out.

Thanks,

Mark

dannyukonxl · August 3, 2004

it sounds like a rich condition (too much fuel) did you notice any decrease in performance maybe, what im thinking is that something along the lines of the o2 sensor or the air injection isnt that the egr? , but anyways somehting that would cause too much fuel to be added, decreasing performance, decreasing running temperature, and therefore decreasing gas mileage,,, just my 2 cents

etex211 · August 3, 2004

Yeah, everyone that I talk to says that I should be having a noticeable decrease in performance, but I can't really say that I've seen that. It will still cruise down the highway at 70 mph with no problem and acceleration doesn't seem to be degraded either.

This truck has 4 o2 sensors (two catalytic converters with o2 sensors in front of and behind each converter). Two of the o2 sensors have been changed in the last couple of years. I probably need to have the codes checked to see if any of those codes are popping up again.

Shouldn't this truck be running at 195 degrees? I'm wondering if someone may have misguidedly put a 160 degree thermostat in there prior to my purchase of the truck.

Also, does anyone know if it is possible to disassemble and clean the electronic EGR valve on this truck?

Thanks again,

Mark

dannyukonxl · August 3, 2004

160 degree thermosdat will cause the truck to use more gas, but my 8100 runs below 210 or 200 forget which one but it runs fairly hot, so that seems fairly low.

i did a search on aol and found this http://www.misterfixit.com/egrvalve.htm at the end it talks about a stuck egr valve allowing gas to be recirculated.. maybe causing the o2 sensors to believe there is a lean condition adding more fuel, more soot in the pipe, lower operating temperature, and as a sum of all those.. decreased gas mileage... it also says egr valves might cost a fortune so i guess id try cleaning it up first goodluck

whitesnake01 · August 3, 2004

When I had problems with the setup of my supercharged 454, I purchased a software product from TTS called DataMater. I installed it on my laptop. It comes with a dongle that goes between your 9-pin and the OBDI under the dash. I can do live and recorded journaling of my engine. You can also record events and then play them back later. With the version I have, it can take up to 10 strobes per second for nearly one hour, recording all litts things like temp, O2 sensor, EGR voltage, etc, etc. It has helped me. While I have not tried, you may be able to get TTS to "diagnose" your engine by emailing them a gzipped log (for a fee).

You can find out more about their products here:

http://www.ttspowersystems.com/

Incidentally, is your maf sensor clean?

whitesnake01 · August 3, 2004

When I had problems with the setup of my supercharged 454, I purchased a software product from TTS called DataMater.

Just noticed that TTS have not updated their DataMaster software for some time, at least as per their website. You probably want to give them a call.

sonjaab · August 5, 2004

Its your EGR valve. Its prob. plugged up with carbon!

GM has updated ones and they do a PCM reflash !

$$$$pendy

My girls 96 Blazer had this happen a few times. A simple

EGR cleaning fixed it up. She had lousy idle and bad mpg

along with a SES light.

Cheap fix! A $6 dollar replacement EGR gasket that has a

screen in it ! Made by Mattco ? Found it on the net!

My local indy. shop keeps a few handy..............geo

Sorry its been a while and I DUH have forgotten the web #

and the full/correct name of the company.

Girl got a new TB last year...................

GMCSierra04 · August 5, 2004

If your EGR is malfunctioning then your gas mileage will suffer. The job of the EGR is to reintroduce exhaust gases into the combustion chamber during cruise speeds so less A/F ratio will be in there. By doing that, there is less heat and the already burned exhaust gases soak up extra heat during the combustion process. It was designed to help reduce NOx (oxides of nitrogen) which are formed in the combustion chamber @ 2500*F and above so it will lower emissions too. So I'd check the valve and see if its cloged up then clean it or replace it.

If your truck is running at 160-170* then its probably never even getting out of open loop. This will give you a rich conditon beacuse the PCM thinks the truck still needs to warm up, thus giving more fuel. At cold temps the fuel would condense on the intake manifold so more fuel is needed to compensate for that. So deffinetly put the stock rated thermostat back in. Good luck.

etex211 · August 6, 2004

Thanks for the replies, everyone. I've got the EGR valve out now, trying to figure out what to do with it. I'll be buying a new thermostat tomorrow.

I'll be using the truck again this Saturday night, so I'll be able to tell if these things help then.

Thanks,

Mark

etex211 · August 7, 2004

Update: I replaced the thermostat and put the EGR valve back on last night. I didn't see any real problems with the EGR valve so I just cleaned it up a little and slapped it back on.

The thermostat that came off looked like the original GM thermostat. It looked like it was not sealing off well when closed. I had that very problem a few years ago on my '92 half ton truck. The symptom that I had on the '92 was that the heater did not work well. With the new thermostat in place, the truck heated up and maintained a temp in the 190-200 range. I am convinced this is going to help.

My big test will be tonite when I make a 240 mile round trip to the races. I'll report back here tomorrow.

Thanks for the help everyone.

Mark