1999 Blazer help - fuel pump going out?

Hi and thanks for any help I can get here. Trying to diagnose this by myself and I have suspicisions that it is a fuel pump problem (probably the easiest fix I could hope for). Sorry this is long but I am not sure if it is a fuel problem so I am going to try to describe what happened. For info it is the 4.3L Vortec V6 VIN W engine, Automatic Tranny (4wd), and has 111,000 miles on it. Been running like a dream until today.

Mileage was average for this at 15-20 mpg of course. I added about 8 gallons of gas today (it was just above a quarter tank full) and right after that it started running - weird. I'll try to explain. I had about a 25 mi trip to take and before I had went 2 miles I noticed it was running rough and gas mileage was showing at 5 mpg. I turned around thinking maybe the gas I had added had water in it and bought some HEET and dumped it in the tank. Didn't help but I headed back on the road thinking it would even out. The RPMs would go higher than normal but it acts like it is bogging down then RPMs would drop and gas mileage would go up for a couple of minutes then it would act like it was bogging down again and it would drop back down to 5 mpg on gas usage. RPM gauge was not showing normal operation since it would go up when I hit the gas but the truck didn't seem to want to respond. It does act like it is not getting the right fuel mixture. I went ahead and drove it the 25 miles and it used at LEAST 5 gallons (or more of gas). No gas leaks or gas smell. Since it was now below half a tank I thought I would fill the tank up with better gas (still hoping it was from water in the gas I bought earlier). Filled it up in the town I was in and it didn't help at all.

I know that I need to check the fuel pressure but first I was wondering if this sounded like a fuel pump / filter problem or not? I know these are notorius for needing fuel pumps. I know these are set to show codes but so far I haven't been able to find info on how to pull the diagnostic codes up. Any help on that?

Thanks.

A bad fuel pump won't effect fuel milage, unless it completely drops out (then milage goes to 0).

Sounds to me like an electrical problem. Either in the distributor area (bad cap, rotor, plug wires), or as you mentioned, a gas problem.

Did the service engine soon light come on?
If so, go to a place like Autozone, and have them read the codes.

Was it raining?

NO SES light at all. Its been moist here for a couple of days but no real rain the day this happened. The mileage was definitely effected since it did use that much gas. That was what made me wonder if it is the fuel pump. I never heard of that particular problem being that bad even if the pump was failing. Any other ideas? It seems like even if the gas was bad diluting it with as much other gas as I did ... it should have resolved at least a little. Limited budget here (out of work and on unemployment in a rural area) so I really need to solve this myself. Any ideas would be appreciated!

It still starts and it idles okay (or seems to) but as soon as you put it in drive it seems to run rougher. You realy don't notice how bad it is though until you drive it.

I found another thread about a blazer with contaminated gas (water in gas) and my tach is doing something real similar. He said in that thread that it ruined a sensor and he (or she?) had to drain the gas tank and replace the sensor (assumption there). How can I check this out for water in the gas?!?! It is a real possiblity since it ran fine till I got gas at our local BP. I really need this vehicle running.

Here's the link to that other post if anyone would like to see it and advise http://www.automotiveforums.com/vbulletin/t907906.html

Thanks for the response.

With no codes, I'd suspect the electrical problem in the ignition wiring, which in my case a similar problem was spark plug wires were shot. Although I did get a P1351 code (High Voltage spikes)

I had bad gas once, truck missed horribly, and mis-firing codes were reported in the SES alert. The light only stayed on for a short period , perhaps 5 minutes, then shut off.

What confuses me is the poor milage you're getting. Even with the poor wires I had, my milage never went below 14mpg (on average, I think you are giving the "instant" milage, correct?).

These things are hard to diagnose without the trouble codes unless you have a good diagnostic system, and it'll cost $$$ guessing what the problem is.

You need to do the fuel pressure test.
If it checks good move on to engine tune up/secandary ign problems.
When was the kast good tune up?
How is fuel filter?

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

If it is shooting that much gas into my system (at 5 mpg real mileage) will it foul the plugs bad enough to tell by checking them? And if there was a lot of water in the gas is the only way to fix it - is to drain out the gas? Pain in the butt since there is probably 3/4 tank still in it. Would water in the gas cause a significant drop in fuel mileage? Hopefully someone that has had this experience will read this. I just don't have the money to take it in. Really need to narrow problem down and fix myself (with the help of fiance and son-in-law who both are pretty good mechanics).

Luckily I have a month before my college classes start before i really need it.

Will work from ignition / electric system down to fuel pressure. Really weird thing is that it still starts up fine and seems to be running okay while idling.

Thanks MT-2500 for the fuel pressure info.

If and when we get it resolved I will post back.

I'm going to add another post detailing weird MPG issue and tach acting crazy. Maybe I can get some more insights.

Most chain type auto parts stores give free code scans and can show hard codes (which turn on the Check Engine light) and "pending" codes, which haven't turned the light on yet.