Real Bad Gas Mileage

[QuakeDeathGod]

I have a 1990 suburban 5.7L 2WD, and after a resent tune up, the gas mileage went WAY down. I was getting around 18 on the highway. Now I am lucky to get 10. Truck is running perfect, no noticable loss in performance or power. In the tune up, I replaced the plugs, (.035 gap) distributor cap, and rotor button, and wires. Changed the oil. Never thought that it would get worse. Any sugestions? Thanks in advance!!

DG

[dirty dan]

First re-check all of your vacuum lines where you were working, especially on your MAP sensor. That along with your O2 sensor decides how much gas your injectors flow. Also check your base timing (don't forget to disconnect the EST wire). Should be at 0 deg @ idle. Let me know if these don't help.

[QuakeDeathGod]

I will look into that now. I am going to get some different plugs for it today and try them. I think I had put autolites in it, I am giong to change them over to the platinumx4 plugs and see it that does anything.
Dont know if this will help or not, one more thing I did to it was add a tran cooler to it. I have a 26ft camper and I am planning on using this truck to pull it. I feel pretty sure that a trans cooler shouldnt have had anything to do with the gas mileage.
But thanks again, I will look at the vac lines, and check the timing and see what I can come up with.

DG

[dirty dan]

That engine takes R43TS AC or Delco Spark Plugs. You're just spending extra $$ for the Plat Plugs, You're really not going to see any difference in performance.
Transmission cooler shouldn't cause a problem with mileage. Too much or too little transmission fluid will cause a mileage problem. OEM says to drive the 700R4 about 10-17 miles before checking fluid level. Could be a problem.

[QuakeDeathGod]

Thanks dan, to late, already bought the plugs. I am going to give them a shot, i've spent more on less if you know what i mean.
I checked the trans fluid, and I dont hear a vac leak. BUT my cruise dont work, it tries but cant hold the speed. That also leads me to think that the vac leak me be a good place to keep looking. But it hasnt worked for a long time. I am going to get really into tomorrow at work and see what I can find when I have better light to work with.
I cant find the EST wire, in my blazer it is under the passanger side floor mat, easy to find. Any one know off the top of your head? I looked in one of those books from advance, and cant find a referance to it. But I know what you're talking about, like i said, I know where it is in my blazer.
DG

[dirty dan]

EST disconnect is a black and tan wire and is located on the engine firewall close to your wiper motor. If you still have the original wire loom there you'll see the one wire come out, go through a connector and go back into the loom. That should be it.

[QuakeDeathGod]

Super, I checked the timing, dead on, I put the platinumx4 plugs in it, and WOW, I can feel a difference. I checked some of the plugs I took out, and the gaps were inconsistant. That is probably where my MPG went to. I should have checked them BEFORE I installed them! And I found a Vac line that is old most likely getting sucked together, it is very nasty looking!!
Thanks a million dan, big help!!
DG