Mileage Drop

I have an 89 C1500 350 with 256k miles. I had to replace the the intake manifold gasket, because water was leaking out and thankfully not in the motor. I had to take nearly every thing off the top of the motor. I was getting around 17 mpg before this happened. It was running bad at first, but I timed it and it is running like brand new. My buddy has a 2002 1500 and it runs as good as his. But now I am getting around 13.5mpg. I use AC Delco plugs and Autozone wires. I recently replaced cap,rotor,wires and the plugs(before gasket). I have always had a flowmaster, but I had to replace my cat. converter and muffler. The guy put a new cat. on - he said that it was the one that they use on the 95+ trucks - also larger diamater pipe from the cat back. It is nothing extreme, but sounds solid - not loud. I also have a throttle body spacer. Could any of these things be effecting my MPG that drastically? Did I mess something up will taking the throttle body off? Any sugestions on what I can check or change?

Thanks - Brian
I love my truck
89 C1500 350

You did unplug the EST wire when you the timing, right? What did you set the timing at? Do you know what it was before you took off the intake?

//2000CAYukon

You did unplug the EST wire when you the timing, right? What did you set the timing at? Do you know what it was before you took off the intake?

//2000CAYukon


I pretty sure we did unplug the EST wire, I didn't check the timing before. We set it at TDC. I should probably check the timing again ,but could the truck be running that good and still have a drop in mileage?

Thanks

TDC is the correct timing with the EST unplugged. Sometimes, if the previous owner replaced the chip with an aftermarket one, the timing may be different.

Make sure all the sensors are plugged in correctly. Especially the temp sensor in the intake. Double check the timing.

I would think that if it is running good, then the mileage should not have dropped.

//2000CAYukon

So...lets see....before you wer'e getting exceptional mileage, and now your'e getting normal mileage.

I have to ask....Did you keep close tabs on your mileage before? Or did you just check it a couple of times?

So...lets see....before you wer'e getting exceptional mileage, and now your'e getting normal mileage.

I have to ask....Did you keep close tabs on your mileage before? Or did you just check it a couple of times?

My dad bought the truck new and I have been talking to him about it and he said that he usually got around 17mpg. I have had the truck for 7 years after him and have never experienced mileage that low. There are several posts in this forum with people who have trucks exactly like mine and are getting around 17mpg. I have driven it at high speeds(85+) while traveling to Colorado from Texas and it still wasn't that low. Confused, but willing to try anything.

theres a prarie dog stuck in your exhaust pipe

When you pulled the intake off did and antifreeze go down the exhaust valve ??? if it did it might had damages the 02 sensor/lazy. Any vacuum leaks at the throttle body?? Cross any plug wires??? any check engine lights on???

When you pulled the intake off did and antifreeze go down the exhaust valve ??? if it did it might had damages the 02 sensor/lazy. Any vacuum leaks at the throttle body?? Cross any plug wires??? any check engine lights on???

I don't remember any antifreeze, but anything could have happened. Is there any way I can check the O2 sensor? No leaks that I can see or hear. No check lights on. Runs perfect. Any suggestions would be very helpful. thanks

a 02 Sensor runs about 20 bucks, it should be replaced when you replace the cat. I would do it just for maintmance purposes.

I have figured it out. After close inspection of my front end I decided to replace it all. I put in new ball joints, tie rods, pitman arm and an idler arm all from Moog. I also replaced my steering box with an AGR Superbox 2. I will soon replace the steering shaft with a Borgeson to hopefully get rid up that 1-2" dead spot. After an alignment I decided to fill it up and check my mileage. I only went 57 miles after I filled up, but I refilled and checked it. 18.22 mpg - better than it has ever been. Half of that 57 miles was in heavy traffic and some of it bumper to bumper. Hope that can help some of you with the same type of problem. By the way my truck drives like a brand new truck right off the line.

so did your ball joints have rivets holding them in? my 2wd chevy does and im worried about having to use bolts to put them back on

As a general rule new ball joints come with bolts- The rivets are just a pain to remove.

so did your ball joints have rivets holding them in? my 2wd chevy does and im worried about having to use bolts to put them back on

I did have to remove the rivets, but it was extremely easy with the air tools. We used an air compressor and a air hammer/chisel. We used the chisel to knock the tops off and the hammer to push them out. It was more or less just a sharp punch that vibrated very fast. Worked in less than 5-10 minutes a side. The hard one was the bottom ball joint. We used to the large C-clamp ball joint press. Major PITA! My quess is that your bottoms are bad. When I took the tops joint out it felt to be in pretty good shape, but the bottom was extremely loose and made much noise. I replaced the bottoms 80k miles ago and already bad. Very happy with all new parts.