Try taking some carb cleaner or an unlit propane torch and spray/flow it around the base of the throttle body just for kicks and giggles. If the idle increases, you have a leak.

 

Still haven't gotten to try that test, but I noticed something else today, when I'm in drive and at a stop, the less pressure I put on the brakes the better the idle is, and if I release the brakes completely from a stop an just let it caost on it's own on flat ground, it idles perfectly, like it now does all the time in park or neutral.

I know just enough about brakes to know vacuum drives them, but I was unable to find any leaking lines anywhere on the motor or brake components. Got any ideas? I'm guessing the leak around the TBI wouldn't do it, but I do know where all that vacuum comes from, so I may be wrong. Any ideas Mr. Admin Sir?

It's also getting really bad mileage I found, 134 miles on almost a tank and a half of gas.

 

pull out the vacuum line from the brake booster. Make note what changes in the idle.

 

Sounds like you have a vacuum or EGR leak somewhere as I stated before. The brake booster does utilize vacuum to supply assistance with applying the brakes, but it should not cause the idle to change significantly when you apply the brakes unless there was a leak somewhere else, reducing the available vacuum in the engine.

But it is also possible that you have a leak in the vacuum booster. Here is the way to test that. Turn off the truck and let it sit for a few minutes. Pull off the check valve from the front of the booster. You should hear a rush of air when you do this. If you do not, you could either have a bad check valve, or a bad booster. If you find that you still have vacuum in the booster with the engine off, I would then do the same test after waiting an hour or so to determine if you have a slow leak.

If you have either no vacuum in the booster or a slow leak, the first thing I would change would be the check valve just because it is cheap. That would be just to rule that out. Then repeat the test. The purpose of the check valve is to not lose the vacuum from the booster while at low vacuum levels during operation or from when you shut the engine off. If you continue to lose vacuum, then you most likely have a booster problem.

As Calvin has said, if you pull the booster vacuum line (check valve and all) with the engine running, you can determine if the booster is good as well. But doing so with the engine running will cause a massive vacuum leak. By covering the end of the check valve with your hand, you can seal the leak temporarily.

 

Just an update I thought might mean something, did some calculations and found that for 6 gallons of fuel I ran 100 miles, but because of the tire size being off it actually ran 93.48 miles which gave me a MPG of roughly 15.58 MPG. That's not bad I guess, not a 4 banger but then agains that the point isn't it.

Also, Wife spent all my money on my birthday(Wasn't that nice), so still working on the vacuum leaks, will post back when I know.

Off topic, but one question, what size fuel tank comes in a 91 4.3 TBI 2wd w/Tahoe package? My Book doesn't say.

 

i have a 94 s10 blazer tahoe 4 door and i have a 18 gal tank and just did my mpg calculations and i am gettin 15.37 miles per gallon on avg (i did this over an entire tank of fuel and went about 270 mi)

another way to check the size of ur tank is run ur gas down to E and then fill it all the way up

 

Yeah well get this, I put in 6 gallons after running out of gas, well it would start and run but going anywhere caused it to die, and then it happened again yesterday at the end of that 6 gallons, so my calculations will be off by roughly 2-4 mpg since i had anywhere from 5 to 7 gallons used. It's frustrating though, started running out of gas the second time anytime i hit the brakes, or rather anytime i wasn't on the gas, it seems the fuel running to the front of the tank did the trick and I was having to restart on the go until i got a few blocks away to the station, but the gauge read a 1/4 right before that started happening and then by the time it was done, about 10 minutes and 2 miles later, it was almost steadily under an 1/8 of a tank.

I just can't seem to figure this truck out. I'm trying to keep it over a 1/4 thanks to a thread about the fuel pump getting hot, but still getting used to having to put gas in every few days. I never fill up tho either, always between 1/2 and 3/4.

That 6 gallons took it from a 1/4 to 3/4 of a tank by the way, so one of three things is wrong, the tank size being 18 gal, the gas pump telling me i put in 6 gal, or the guage reading that i had 3/4 tank of gas.

 

Alright, sorry for the long delay. Got an o2 sensor and some carb cleaner today, though didn't use the carb cleaner yet, or the can of seafoam I got.

I did buy a Bosch sensor, but I know what's wrong with that brand, still trying to figure out what's wrong right now so any improvement is enough, I'll get ac delco later in a few months. The truck idles MUCH better and in general runs better, so it looks like the os sensor was the problem, drove it for a while but will find out tomorrow on a longer drive in heat if the cel comes on again, it didn't tonight.

I will also check the base of the tbi for leaks, and I'm wondering about the seafoam stuff. Says it treats up to 10 qts. of oil for one can, so should I use the whole can or just half of it since the truck only holds 5 qts.? And yes, I am going to put it in the oil first, already got my second bottle of injector cleaner over four or five tanks in the gas right now, actually it's octane boost this time, same difference though. Anything you know that I should know about it? Also, just ahd the oil changed less than 300 miles ago, so don't wanna do it again if I don't have to, but after putting the seafoam in don't I have to fairly soon afterward?


And still kind of on topic, found a wire that is unhooked todaym, it is tied in with the main harness on the firewall behind the tbi, the wire appears to be black or some dark color and has a single terminal on the end with a rectangular pastic housing on it. I couldn't find anything to hook it to, do you have any ideas or would a picture of it be more helpful?

Thanks

 

hey was wondering did the o 2 sensor solve your idling issues?

 

Mostly, Yeah