Thread: in town fuel economy

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Jerry,
Interestingly enough now that you mention it the economy does not seem to change appreciably between winter blend and summer blend and even factoring in foothills and traffic when you are putting in 20+ gallons of gas every 175 miles, give or take, there is a problem somewhere. I have been driving these roads since 92 with a variety of vehicles and this is the first one that is this thirsty. There is fuel economy in these engines somewhere, if I am not mistaken the hot rod Corvette gets good enough mileage that it is not subject to the gas guzzler tax. Granted there is little comparison between the Corvette and my truck but it is still a gen 3 small block.

Cleaning the injectors can't hurt. I supervised an LAPD repair shop in the 90's, I rarely did any work on the cars but as I recall it is just a matter of attaching a device to the fuel line and screwing a can of cleaner to it. Any idea if any of the chain parts stores rent the equipment?

As for dyno tuning ask Corey about the E3 plugs, do they really live up to the hype or is it just advertising smoke? Also what about an aftermarket tuner, can I tweak and peak the timing and mixture with one of these. While I am not totally opposed to pulling the engine and playing around with camming and heads it is not at the top of my to do list. If there is something that I can do externally to get the economy into the acceptable range it would be the preferable solution. Thanks for the offer.

The more I think about this the more I am leaning toward a mechanical problem not electronic. My scan tool shows nothing and the truck recently passed smog. I know for a fact that a vehicle will not pass smog if anything is even the slightest bit out of factory parameters. I even had a vehicle fail because it had a hard code set for the knock sensors that would not cancel. It was an Infiniti and I needed to take it out and drive it on the freeway for about 50 miles to get the code to cancel and then go back for a retest.

Soooooo I just bit the bullet and ordered a new set of injectors, not reman not aftermarket, and the upstream O2 sensors. As someone in this thread observed they can coke up and send false information. The downstream sensors are only checking cat efficiency and have little or nothing to do with engine management. Parts should be here next week. I will get them in and report back after a tank or two.

Thx for the O2 sensor tutorial. Educational.

I do know that the sensors are working because winter has finally arrived in So Cal. When I leave the house in 50 degree weather the truck runs like it is about 10 degrees late in timing until they warm up and it goes into closed loop. How well they are working I hope to answer later in the week.

OK so I finally got the injectors and O2 sensors in. I started and when I looked under the hood I realized that it probably would be a good idea to have the o rings for the crossover tube and return line. I tried to get them from the local dealer and they needed to order them so I just found some on Amazon for less than half the price and ordered them myself. But I digress. We have been experiencing a very strange phenomenon here in southern Cal, someone told me it is called rain. Haven't seen it in so long I forgot what it was. Anyway I was having a problem finding dry weather and the time to do it happening at the same time but did prevail. With the new parts in, the truck does indeed run a lot smoother. I have only driven it a couple of times but I was in a parking lot with the engine running , trans in park and was making some notes. I thought that the engine had died until I looked at the tach and saw that it was idling at 500. It was running so smooth and quiet that I could not feel it or hear it.

When I get a hundred miles on it I will stop and fill it back up and see what happens. Here's hoping.

Also I found out why it runs so sluggish in open loop. The timing does not advance until the system warms up and goes into closed loop and then it will advance properly. Is this normal or do I have a computer glitch which could explain a lot.

It does have heated sensors and goes into closed loop within about a half a mile but it seems odd that it would kill the advance in open loop. I have never noticed that on any other vehicle but admittedly I have never looked for it either.

I understand everything you are saying. All that I am saying is that it is going into closed loop, but before it does the ignition advance is not what it should be and I found that strange. But it does make sense that as most of the system is "off line" that the pre-sets are rather conservative to prevent possible damage. Driving it with the scan tool on the O2 sensors are not active initially and all the read outs are pretty stable. As soon as the O2 sensors come alive all the readouts change and move around as they should.

I do have a new problem though. When I put in the new injectors I noticed that the vacuum tube to the fuel pressure regulator had a tear in the upper connector. I assumed that I had damaged it when I removed it but if it was torn before I started the fuel pressure regulator was not getting a vacuum signal which is probably adequate explanation for poor low speed economy. When I drove it I was taking care of a couple of chores. Made a stop and when I came back about a half hour later the engine was reluctant to light. it cranked for about ten seconds and finally reluctantly picked up all cylinders and then ran fine. Made a second stop same scenario. IF the vacuum line was damaged before I started working on it that would mean that the fuel pressure regulator has not been off of its seat for a very long time. I seem to recall that the fuel rail is supposed to maintain pressure, so what is the possibility of the regulator now being reluctant to close, thus leaving the fuel rail with no pressure when stoped. It would take the fuel pump some time to pressurize the rail again with an unseated pressure regulator possibly explaining the slow start.

If I pull the regulator off and connect it to a hand vacuum pump are the mechanics visible enough to see if they are moving properly?

Now you are trying to make me computer literate, it is a lost cause. At my age it took me a couple of days just to figure out how to turn the d.... thing on.

I find it a whole lot easier just to call my computer guy, have him log on to my computer and make it happy.

With regards to the truck. Been driving it around for a week or so and put 98 miles on it. Stopped for gas, same pump at the same station, and put in 8.4 gallons. That is a dramatic improvement over what I had. I was filling up 20+ gallons at 175 to 180 miles. It may still improve marginally as I have noticed that the throttle response is improving as the computer learns. I believe that the vacuum line to the fuel pressure regulator was damaged before I worked on it causing full pressure in the fuel rail which would be reason enough for the poor economy. The new injectors and O2 sensors probably did not hurt but bottom line is that it is dramatically improved over what it was.

I still have the starting problem but it only happens if I stop when the engine is at operating temperature and the truck sits for 45 minutes to an hour. It will take around 5 seconds of cranking for the engine to light. If the truck cools off completely first or second piston up will light and the engine runs fine. I haven't had time to wrap my fingers around a fuel pressure gauge to check for bleed down but I still wonder if it isn't the pressure regulator sticking from long time lack of movement. That will be my next task when time permits. It is not critical as the truck is running fine and economy is up to an acceptable level for now.

It sounds reminiscent of heat soak, when the fuel boils out of the carb.

Bleed down sounds likely.

Got my fingers wrapped around a fuel pressure gauge and checked on a hot engine. KOEO 49 pounds. Running 44 pounds, I pulled the vacuum hose off and it jumped back up to 49 pounds. Shut the engine off and in a couple of minutes it crept up to 55 pounds. I am guessing that it was the fuel heating up in the fuel rails. Within 45 minutes it had bled down to 0. Not sure what spec is but as I recall the police chevy's with TBI ran on 35 pounds and the ones with port injection ran on 50-55 pounds. Ordered a new regulator should be here early next week.

OK. I hate it when I follow a thread all the way to the end and there is no conclusion so here goes. At the beginning of this thread I was fueling up every 150 to 200 miles 22 to 24 gallons. The last tank I went 293 miles on 23.6 gallons, 12.42 miles per gallon. I believe that I can state that this is the first time since I have owned this truck that it went any where close to 300 miles on a tank around town.

The largest change was installing new upstream O2 sensors and a new set of injectors. They went in at the same time so I do not know which of the two made the change or if it was a combination. I also found a tear in the vacuum tube to the fuel pressure regulator which I replaced at the same time. The fuel pressure regulator had no vacuum signal for an unknown period of time and when I put vacuum to it the diaphragm ruptured bleeding a small amount of gas into the plenum. This created a hard hot engine start as it was in effect flooding the engine by filling the plenum with raw gas fumes. The running fuel pressure was low also. It was running at 43 psi and with the vacuum signal disconnected it would jump up to 50. I ordered a new regulator and put it in. I rechecked the fuel pressure and it is now running at 51 psi and with the signal tube disconnected it jumps up to 60 which as near as I can tell is in the range it should be in.

The tank of gas mentioned at the beginning was after installing the O2 sensors and the new injectors but before the new pressure regulator. The pressure regulator went in somewhere in the top half of that tank and for the most part cured the hot start problem. Every change I have made has resulted in the engine running differently until I got enough miles on it for the computer to start re-learning. Right now the truck runs fine but it still occasionally is a little slow starting if I shut it off with the engine hot and leave it sit for about an hour. When it does happen it is the exception not the rule so for the time being I am tired of looking at an open hood and will let it go to see what happens.

I will continue monitoring the in town mileage for a couple of tanks and if it changes after the computer completely re-learns I will update the miles per gallon.

I still haven't ruled out a cam change if I can find someone that will grind what I want.