TBI - sudden mpg drop

[rlb]

Trying to figure out what could be causing a sudden drop in the MPG of my truck. 94' TBI 5.7L 4x4 2500 suburban

About two weeks ago I noticed a hesitation/rough running right above idle throttle. This was preceded by a check engine light 44 - lean exhaust condition indicated. Two days later when I go to troubleshoot, the problem is gone, the truck is running great. No check engine light, no roughness.

It is still running fine, except the MPG has gone to hell. I filled up, drove 90 miles and burned 11 gallons. This was all highway at 65mph. I have measured 15mpg before on this trip!

Not sure exactly where to start. Less than 3000 miles ago I put new injectors in the throttle body, did 3 wire O2 sensor, couple other things and after that work it was getting a nice 15mpg!

 

[WarWagon]

Check all hoses esp the one going to the MAP. Suspect you need to replace the EGR valve with a AC Delco unit if you haven't already as these randomly flop wide open when failing as soon as the vacuum hits them just off idle. Electronic ones just stick open - no codes. Possible brake booster has a leak.

The TBI is s stupid simple system that mainly uses map and TPS to calculate load aka fuel need. Your TPS is a wear item so check it with a meter.

Also look at the fuel pump pressure and replace fuel filter. Note you replace the fuel pump relay with the pump period.

 

[rlb]

Thanks. I will check EGR first. I haven't touched it as I figured it would throw a code if it was bad. I have a service manual and will consult for a way to test the TPS and EGR.

Already checked all vacuum hoses carefully to make sure they are intact. Brake booster is hydro.


Sent from my iPhone using Tapatalk

 

[rlb]

Video I took when it was running rough last week. It does not run rough like this anymore, note my original post.

Sent from my iPhone using Tapatalk

 

[THEFERMANATOR]

Check the throttle body base gasket. These commonly blow out, and cause all kinds of problems on the TBI.

 

[udidwht]

Also check the cooling temp sensor.

 

[MrMarty51]

Plugs, plug wires, rotor, distributor cap. None of these will trip a code but they’ll sure out Your fuel mileage.

 

[Will L.]

Well, is it still running good but bad mpg for the last 6 years since posting?

 

[smokymtn65]

newbe posting to all old post lol

 

[Will L.]

@udidwht
Welcome to the forum. Like to see when someone new enjoys helping out others.
And checking the dates first might be beneficial

 

[MrMarty51]

And I go and reply to the old threads.

 

[udidwht]

It may be old but still applies and may help one down the road. Nothing worse than starting a new thread when dozens or more already exist and can continue to provide useful info.