99 7.4L K3500 - Losing power and mileage

This has always been a great place to come for good information and I'm hoping this time won't be any different. ;)
I can generally get around 12-13 mpg with pure interstate driving (no
big headwinds or anything) from my '99 K3500 with the 7.4L engine. This past week I needed to drive about 250 miles on mostly interstate with an empty car-hauler type trailer, pick up 20 school desks (total weight couldn't have been more than 400-500 lbs.) and return home. Before leaving, I checked all fluids and everything looked normal.
About halfway into the trip I filled the tank and noted we had been getting a little over 11 mpg. This is exactly what I expected at that point, so the trouble must have started after.
Arriving at our destination, the first thing I noticed was that the engine was idling a little slow and a little rough. Also, there was a serious lack of acceleration from a standing start. I thought maybe we'd gotten some bad gas. When we stopped to load up, I opened the hood and there was a strong electrical smell and a slight odor of gasoline. This past winter we had a mouse build a nest on top of the plenum, and my wife thought it smelled more like mouse nest to her, like maybe we hadn't gotten it all out. I left the hood open to let it cool better and air out some.
After we loaded everything up and got it tied down, we took off. No change in performance. On the way home we DID have a bit of a headwind, but nothing too serious. We stopped to make sure everything was riding okay and fill up. Now we're getting about 7 mpg.
This truck would ordinarily pull this trailer like it wasn't there, even with 400-500 lbs. on it, but now the transmission would shift down every time we got to the slightest grade.
The last time we filled up, we were getting just over 6 mpg.
The "Check Engine" light doesn't come on and I don't have a scan tool.....yet. I've ordered one of these units that hooks up to a laptop so I can check all the sensor readings and see if there are codes stored that don't activate the CE light. I don't actually know if that's possible, but I've been told it is.
On a near-full tank, I put in two bottles of fuel system cleaner w/Techron - not a double dose, but each bottle only treats up to 13 gallons or something and I've got a 34 gallon tank. There was probably about 26 gallons in it when I added them. I've run it some since then to give the cleaner a chance to work through the system. I still get the same odors from under the hood - electrical and gasoline.
I also have a slight coolant leak from the intake manifold gasket which - thankfully - is leaking exclusively to the outside.......so far.....
I'm figuring on replacing the intake manifold gaskets, which will require R&R of the plenum, which will give me access to the fuel rails and injectors and wiring.
I'm open to any and all suggestions here. One thing I DID do was disconnect the battery cable for a few hours in an attempt to reset the computer. No change in performance or symptoms occurred.
Thanks to anyone who has any ideas.
JLarsson
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wrote:

I think the "cure" here is easy, use higher octane gas. GM uses very aggressive ECM spark control to control knock before you here it to limit consumer complaints. The "problem" is that when you retard the spark, you take a big hit on MPG and power and it can make engine feel hotter too because the exhaust will be after burning some in exhuast manifolds. Though some people what to believe otherwise, 87 octane is not enough for a modern 454 towing. ----------------- The SnoMan www.thesnoman.com
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SnoMan wrote:

Thanks for weighing in on this SnoMan. Octane was something I hadn't really considered since I've used 87 in this exclusively since purchasing it used about 4 years ago. And if it were the grade of gas used for towing, wouldn't it clear up when I took the trailer off? As it idles - even now, days later and from a fresh start with nothing hooked up to it - there is a slight abnormal vibration that suggests missing, yet no CE light.
I'm game to try the better octane fuel - what would you think if that turns out not to be the solution?
And thanks again.
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wrote:

WHen a engine retards spark a lot under load it can foul things up a bit as the heat will be dumped in exhaust and plugs can load up and explain rough idles for a bit afterwards. Also I want to stress that if you are suffering from knock/spark retard, it can be very hard on exhaust valves over time if engine is working hard. You might trry a tank 89 or if you have a 1/2 tank now, top it of with 93 an that will give you a mid grade or better tank full and run it for a tank and see what happens. The octane requirement of a engine actually tend to increase with age as deposits build up in chambers and engine use small amounts of oil too and the presence of oil in even very small amounts can lower the octane of fuel. ----------------- The SnoMan www.thesnoman.com
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SnoMan wrote:

I see. This engine HAS always used a bit of oil between changes. 1/2-3/4 of a quart usually.
I'll try some of the higher octane gas. We'll see what happens. Thanks again for the help.
Thanks to you too, Dave. Lotta' things there to check out. Ought to keep me busy for a while. :D
JLarsson
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My EGR was not working(needed to be cleaned) and the first thing I noticed was that the truck was running real hot(about 220ish towing nothing on a short drive). Part of the point of the egr if i understand it right is to reduce the combustion temps in the engine when going at a steady rpm(highway driving). That being said, it did throw an intermittent check engine when this happened.
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The theory behind a EGR valve is to delute mixture a bit so that it burns a little slower and cooler to reduce NOx emissions and to reduce HC's by afterburning in exhaust a bit, not to reduce engine temps. ----------------- The SnoMan www.thesnoman.com
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SnoMan wrote:

Thanks for weighing in on this SnoMan. Octane was something I hadn't really considered since I've used 87 in this exclusively since purchasing it used about 4 years ago. And if it were the grade of gas used for towing, wouldn't it clear up when I took the trailer off? As it idles - even now, days later and from a fresh start with nothing hooked up to it - there is a slight abnormal vibration that suggests missing, yet no CE light. There is still a serious lack of power, as well.
I'm game to try the better octane fuel - what would you think if that turns out not to be the solution?
And thanks again.
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let me see if I've got this right, after a long steady state cruise you experience rough idling poor performance and increased fuel consumption? During steady state cruise the EGR system actuates and the catalytic system is operating. also read this page http://autorepair.about.com/cs/troubleshooting/a/bl303a_5.htm
Good luck.
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Okay - a brief update - I'm not quite down to half a tank yet, so I haven't filled it up with whatever the premium grade is locally.
But there IS progress! Of a sort, anyway.
SES light is now lit. So when my scanner comes, there should at least be a code of some kind that I can read.
I figure one of two things is probable, although more possibilities exist: 1) Whatever is causing the problem was going bad fast enough to cause a performance problem, but not extreme enough to throw a code until today, or 2) Whatever was wrong has now caused something else to fail. I'm thinking it might be a good idea to keep this vehicle parked until I can read the code.
Any other suggestions? And thanks.
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wrote:

The type of code would etermine whether to park it or not. Some codes can be quite minor. Last year I bought my daughter a used Saturn and it was throwing a few codes related to misfiring (it was running fine though) but when the thermostat was replace as it was bad, it threw no more codes. The codes can be a big helps at times but somethimes not too. ----------------- The SnoMan www.thesnoman.com
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SnoMan wrote:

Yep - I agree. But without knowing which code it is right now, I'm more prone to park it.
Thanks again.
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Keep in mind That most of the US has switched over to Ethanol. Ethanol has the unique ability to loosen rust off the inside of tanks. (Thats why there was a idle of storage tanks, while they were scrubbed) This my be the cause, and is worth looking into.
This might just be the case of a clogged fuel filter.
Cheers

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Okay - here's an update.
Got my scantool today and hooked it up to my laptop using ProScan 4.0a software. Very interesting and great to have all that info available in real-time.
Only stored code was P0171 - Fuel Trim System Lean Bank 1. Per the "live" sensor readings, Long Term Fuel Trim is listed (at idle) at around 18% for Bank 1, -5% for Bank 2.
Any ideas on where I should start in diagnosing this? Related to that, where can I find the "normal" output values for the various sensors?
Filled it up with premium two days ago and looking to see if that changes things. Still seems like I've got something more hardware oriented.
Thanks again to all.
JLarsson
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Well, I may be beating a dead horse here, but......
A brief update. All four of my oxygen sensors are identical. So I thought I could confirm or eliminate the 02 sensor as the cause here by switching the left and right sensors to see if I could transfer the problem to the other bank (bank 2). Switched them and it made absolutely no difference. Same readings all around.
I'm less inclined to think it's the MAF sensor, the fuel filter, or the fuel pressure regulator because the problem is specific to bank 1. I guess that leaves injectors, vacuum leak, or.........what? Spark plugs? Wires? Distributor cap or distributor?
What would your next step be? And thanks.
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