EVAP Hose Issue

[scholzee]

OK went outside and looked that hose goes from the charcoal canister the the vent solenoid mounted on the rear crossmember by the gas tank. If the hose was kinked closed the problems it would create are check engine light for failure of evap test, a sound of air rushing into the gas tank when you open the gas cap, possibly low fuel pressure due to build up of vacuum as fuel is pumped out of the tank and air not being let in. that big of a hoes is tough to seal with a kink like that, so I kind of doubt it was giving you trouble unless one of the above was going on.

[scholzee]

To answer you question specifically it lets "AIR" into the fuel tank just like its labeled. If you use a gas can to fill, say a lawnmower, the gas comes out slow due to vacuum build up unless you open that little vent hole on the can. That hose lets air in when the solenoid on the crossmember is opened.

[JoshBarber]

Originally Posted by scholzee

To answer you question specifically it lets "AIR" into the fuel tank just like its labeled. If you use a gas can to fill, say a lawnmower, the gas comes out slow due to vacuum build up unless you open that little vent hole on the can. That hose lets air in when the solenoid on the crossmember is opened.

well I was having two fuel related problems, and I wondered if EITHER of them would have been caused by this:

a. loss of MPG, and surging during acceleration

b. hard to start after vehicle has sat for an extended period of time.

[scholzee]

Sorry if I missed it, but what year,how many miles, and how bad of MPG on your vehicle. I have a 2dr 4x4 1996 with 200K on it and consistantly get 18.5 mpg with 75% highway drinving at about 65 mph and 25% city driving. My hard to start truck was fixed by a new fuel pump, but I did many tests before I changed it. there are different " fuel injection " systems and they changed in 1996, before then 1995 and later used CPI which has a tendency to leak in the intake manifold and give problems like you describe. You also mention a fuel smell which can be an external leak possibly causing low fuel pressure and surging, but I would think you would see some dripping somewhere. We need more details.

[JoshBarber]

Originally Posted by scholzee

Sorry if I missed it, but what year,how many miles, and how bad of MPG on your vehicle. I have a 2dr 4x4 1996 with 200K on it and consistantly get 18.5 mpg with 75% highway drinving at about 65 mph and 25% city driving. My hard to start truck was fixed by a new fuel pump, but I did many tests before I changed it. there are different " fuel injection " systems and they changed in 1996, before then 1995 and later used CPI which has a tendency to leak in the intake manifold and give problems like you describe. You also mention a fuel smell which can be an external leak possibly causing low fuel pressure and surging, but I would think you would see some dripping somewhere. We need more details.

98 Blazer LT 4x4
approx 123,000
MPG was approx 16-18mpg and is significantly lower recently

How did you determine the fuel pump was the culprit?

[gmtech79]

Originally Posted by JoshBarber

what do you mean? somethings different on mine?

Yes, that is not the factory hose. It is originally a plastic hose like your fuel lines and those other evap lines are. Where is your vent valve at now?

[JoshBarber]

Originally Posted by gmtech79

Yes, that is not the factory hose. It is originally a plastic hose like your fuel lines and those other evap lines are. Where is your vent valve at now?

take a picture of yours, cuz Im not following

[scholzee]

Mine would have trouble starting after sitting a few days, there is a check valve in the fuel pump tp keep the lines primed with fuel, when it goes bad it takes time to refill the system leading to longer crank times. I am not saying to replace the fuel pump but it is very sensitive to battery voltage to get the proper fuel pressure. The injectors will not squirt fuel at marginal fuel pressures. Sometimes the battery voltage gets low from cranking and the fuel pressure gets lower and it will not start, someone jumps it and the extra battery gets voltage up fuel pressure up and all of a sudden it starts. here is some stuff on the fuel pressure.

Central Sequential Fuel Injection (GM Trucks)

Central Sequential Fuel Injection (Central SFI) is used on 1996 and above light trucks to meet the increased emissions requirements. Central SFI is used on all 1996 truck engines except some carryover engines in G vans and P/G units over 8,500 pounds. This system is very similar to the previous CPI system used on the 4.3L V6 engine. The major difference is that in lieu of a single injector feeding all cylinders, the new design uses a single injector unit for each cylinder poppet nozzle. Each is then fired sequentially for precise fuel control.

Overall, system operation is very similar to the previous discussed CPI unit with fuel pressure being super critical. The fuel pressure specification is 60-66 psi KOEO. As mentioned, the fuel pressure is very critical for proper operation and may be somewhat misleading when a vehicle is close to specification. Close won’t work on these applications!

Fuel Flow Test at Different Fuel Pressures
Flow test is from left to right:
54 psi = 37 MILS
56 psi = 41 MILS
58 psi = 44 MILS
60 psi = 49 MILS
62 psi = 53 MILS
64 psi = 60 MILS

As shown above, for every second psi change in fuel pressure, the actual fuel delivered to that cylinder changes about 10%. As we have said in the past, fuel pressure is very critical. Our test unit (new OEM) is regulated to maintain 64 psi with some return beginning at 62 psi (KOEO). The poppet nozzle will begin to spray fuel at 48 psi (a poor stream at this pressure). Any pressure below 48-50 psi, this engine won’t even spray fuel at all.

As far as the bad mileage or fuel smell I will try to find some stuff on the fuel pressure regulator. To test fuel pressure there is a shrader valve " like a tire valve stem " back on the top of engine near distrubtor, a fuel pressure gage sscrews on to it to measure pressure.