gas MPG drop

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Over the last couple weeks I have been trying to figure out why my MPG has dropped on my 1996 chevy k1500 with 230,000 miles. At first I pawned it off as a bad tank of gas but 2 tanks later it didnt get better. I went from getting my usual 17-20 MPG to only getting 10-13mpg. It seemed to have happened more of suddenly. I have also noticed that my truck has had to turn over more than usual to start.

I have watched most of eric’s videos on this and I have been watching my O2 sensors. the front ones seem to change pretty good from what I can tell, however I don’t know how many miles are on them or if they have been replaced ever. the rear ones that measure the catalytic converter seem to be all over the place especially on bank 1. They will stay at a fairly constant high 65-70 for a little bit and then switch to a low number 9-18 and change back and forth between those numbers with a constant 2000 rpm.

I have also been watching my fuel trims. at idle it will reach -10 even after the truck has warmed up. as I start going down the road the trims get better but still stay around -5. the truck was idling at 800-1000 so i checked my coolant temperature and it was saying that my truck was only running at 179 degrees when it has a 195 thermostat. so I replaced the thermostat and it now runs at 193 degrees and idles around 650. but it still is running rich.

I did a compression check and got 140-145 on 4 cylinders I have not had the time quite yet to get to the other 4 but I will ASAP.
I havent noticed a real loss of power if I get on it, but I have noticed that where it used to go 65mph at 1700-1900 rpm it now takes 2100 rpm.

At Idle it does have a slight shake and as the rpms come back down from being reved its almost like it gets choked for a second.

I was questioning maybe my catalytic converters were the cause so I crawled under the truck and examined the o2 sensor wires. I also unbolted the exhaust and I could see one cat and it looked kind of a yellowish color and in a healthy honey come shape. I couldnt see the other cat but when I give it a good whack it sounds like something is loose inside almost like a powder nothing big or heavy sounding but there is something loose in there. I isolated the heat sheild the best that I could but I’m pretty positive its coming from inside the cat. I know that for more than 3/4 of the trucks life it was ran on 85 octane gas and didnt get a tune up untill it couldnt hardly run. After I did a tune up it was a beast.

I did get new tires recently however this issue did not start untill about 3 weeks after. I have also double checked the tire pressure.

the truck has:
new distributor shaft, cap, rotor, wires, platinum plugs.
clean k&n air filter.
new fuel filter
new fuel pump
new thermostat
the cel light is not on

I’m not really sure what direction to go from here. I want to replace the o2 sensors and cats but not if they arent bad.

Topic

[Andrew Phillips]

My knee-jerk reaction to your condition is one or more leaky fuel injectors. Especially with no codes set, your description of prior poor maintenance, and the post-cat O2 behavior you described. A leaking injector would also cause the fuel trims to go negative as the computer is trying to compensate for the extra unmetered fuel leaking from the injector(s). This is even more plausible as you say the fuel trims become less negative as the engine calls for more fuel, it doesn’t have to take away as much fuel as it did at idle.

[Tanner]

I’m not sure why that didn’t cross my mind but thank you. I took the intake off and looked into the throttle body with a flashlight and I can see two fuel lines one to bank one and the other to bank 2. And both were wet and I could smell gas. There were also 2 or 3 small puddles of fuel on the bottom of the intake. The vortec has the fuel regulator and injectors under a plastic cover under the throttle body. So I’m guessing that’s probably my best bet of the problem. But is there anything else I should look at?

[Andrew Phillips]

There are probably several other things, but it looks like you found the source. Take care of that first before looking for other things.

[Tanner]

I decided to hold off on the injectors just because they are like $300 for my truck :/. I got a fuel pressure tester that was on sale at my local napa. my fuel pressure is in spec and the pressure didnt bleed down. but one thing that i was wondering about. wouldnt that just diagnose if the regulator was leaking or not and not the injectors?

I replaced my front O2 sensors. one had 230,000 miles im guessing on it because it was original equipment and wasnt the updated part. and the other was a different brand with who knows how many miles on it. I repaced them with GM original ones. I also replaced the coil that tested out of spec on resistance.

my truck has better acceleration and slightly better mpg but its still way rich and it seems like the shaking at idle has gotten worse.

I did notice that around the egr valve it had a liquid coming out of it. that should be sealed off shouldnt it? could that make it rich? if it was a vaccuum wouldnt it be lean??

[Andrew Phillips]

I still suspect you may have an injector issue. They do not have to be leaking to still contribute to fuel problems. They can be plugged or sticky as well, causing improper spray patterns, which can be almost as bad as a leak. You do not have to replace the injectors unless the coils go bad and they don’t open. They can be cleaned and overhauled rather easily. There are places you can send them to, or maybe one in your own town, that can clean them for you for about $20 each. This service usually also gives you a before and after performance report.

What are your current fuel trims, if you still have access to a scan tool?

Did you smell the liquid coming out of the EGR to see if it had a raw fuel odor? Was it thin like fuel or thick like oil?

Did you manually test the EGR to see if it opens/closes? Is it stuck shut? Hard to move?

[Tanner]

I decided to clean my air flow sensor because i recently cleaned and re oiled my air filter. it didnt seem to make much difference but while i had it all apart again I decided to check out under the throttle body again. this time it smelt pretty strong of gas and I looked with my flashlight again. only this time i looked more at an angle towards the firewall. it looked alot wetter to the point that i could see silver parts of the intake.

So just for the heck of it i decided to hook up my pressure tester gauge again and see what happened. It pressured up like normal and while i was waiting to see if it lost pressure something started randomly clicking. it would do it in bursts and then stop for a time and then do it again. as I watched the gauge with it clicking when it clicked the pressure would go down. the only thing that i can think of is that my injectors are opening when they are not supposed to.

From what I understand the poppet valves on this system open and open and close with pressure. GM put out a updated version that makes it more of a mpfi with electronic injectors instead. the injectors are under the top part of the intake manifold. this upgrade is supposed to be much better than the poppet valves and if thats what i need to do I would rather just replace them mostly because it seems like a lot of work to go in and just replace one smaller item and have the chance of haveing to do it again.

My fuel trims are about the same. at idle the short term trims climb as high as -11 as well as the long term fuel trims. if I am going 30 or 40 mph down the road the short term are around -5. and at 65mph the short term trims are 0-4 almost always negative hardly ever positive. It seems the slower I am going the more gas it has to take away. the faster Im going the better it gets.

I am using a scangauge 2 so I always have access to the trims and other info. the trims also seem worse if its a little colder outside.

I looked at the EGR valve and I think the liquid may have just from when i changed the thermostat.

[Tanner]

I finally got the new injectors and installed them. and you were right my old ones were leaking in several places and the injectors also seemed to not be able to spray very good. my mpgs are back and my truck has more a lot more pep. thanks for the help

[Andrew Phillips]

Glad it worked out for you. Thanks for posting your result. Sometimes we give out advice and never know what happened or if the issue was fixed. Cheers.

[Author]

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