Hello all, New to this forum. I just bought ($100) a 98 Blazer with 208k miles, which will not start 98% of the time on it's own unless you pour gas into the throttle body. I have gone through the sticky on this site and have done all the checks I can considering I do not have a scanner. After it starts with the gas poured in the engine revs well if the pedal is slowly depressed. If you jump on the throttle it stumbles badly the slowly works its way up to about 4000 rpms. No trouble codes or check engine light. All fluid levels are on full.

Here is where I stand: 1. I changed the fuel filter and hooked up a pressure gauge. mid 40's as soon as you turn the key on and does not go down. The pressure once it starts is mid to high 50's and stays steady. 2. I cleaned the MAF sensor and at first it started better, but then it went back to not starting. I replaced the MAF and it made no difference. 3. Just had a Cadillac that would not start and the fix was to change the crankshaft position sensor. I changed the crankshaft position sensor but no change. 4. Checked ignition wires for spark while cranking and had strong spark.

PLEASE HELP!!!

Thanks, Andy in Kentucky

 

The fuel pressure you quoted is no where near spec as detailed in the fuel pressure testing article (did you read that one?). Needs to be >55psi at key on, engine off and not drop below 50psi for 10 minutes after the pump shuts off. Since you don't ever reach 55psi, I'd say that your problem is fuel system related.

 

Thanks for the reply. I did read it. The fuel pump produces 55+ psi once started. Not sure why it does not immediately when the key is turned on, before cranking. I suppose that the pump only runs for a timed period and not by pressure cutoff. Can't think what would be the cause since the problem since the pressure immediately jumps to spec once the cranking begins. The Chevy 4.3 arena is all new to me. Is the crankshaft sensor or the camshaft sensor used to tell the engine is cranking in an attempt to start? Usually the crankshaft sensor does that function by who knows, maybe Chevy uses other means.

 

The diagnosis thread will walk you through the process of identifying the source of the low pressure. If the pressure does not exceed 55psi with the key on, engine off and doesn't bleed off, the pump is likely weak and needs to be replaced (as the thread details). Once the engine is started, there is more voltage running to the pump which would tend to mask a weak pump.

 

It does not matter in your case as far as the fuel pump and pressure. Ign yes but fix the shortfall first.
You're not getting the message. The fuel pressure test is very specific for a reason. The reason is this particular fuel delivery system is touchy.

The reason it's touchy is that GM was testing out fuel system design options in this engine series.
The design sux, IMO, but things go a lot better in the reliability and 'keeping it running' dept if you just accept and adapt to it... rather than should'as and coulda's

 

The more I look at this fuel system (especially the fuel injection spider) the more I tend to agree that this fuel system Sucks. What a mix of throttle body injection with a multiport mixed in?? I have found out that a majority of aftermarket companies do not make the GM style system. They make a conversion kit with individual electronic injectors for each cylinder which still looks like a spider.

swartlkk: I tend to somewhat agree with your theory about the fuel pump possibly being weak. I helped to start the truck a couple of times when hooked up to my F350 (running) but it went right back to doing the same old thing after two starts no matter how long I let the battery charge. I will go back and try that again and see if I get more fuel pressure when jumped, key on engine not running in the Blazer.

 

There is no comparison between TBI & the SCFI system. If you were talking about the older CPI system, I would agree with you. All GM did with the SCFI system was house the 6 individual injectors in a central metering body. The downside of this was that they put pressure actuated poppet nozzles at the end of the lines that the electronic injectors feed. These nozzles gum up and either start to leak a bit or don't open at all which causes misfires, etc and gets away from the true sequential nature of the system. With the upgrade to a true MFI spider (as offered in the mid-2002 on up s-series & fullsize 4.3L v6 engines), the poppet nozzles go away and the electronic injectors move to the intake runner.

I am pretty fairly certain you have a fuel pump issue. The dismal pressure with the key on, engine off is a dead give away. Especially if the pressure doesn't bleed off. You can confirm this by installing the gauge at the fuel filter with a shutoff valve behind it to eliminate the engine/return side of the fuel system. The pressure should well exceed 60psi at this location.

The only other possibility is a bad ground for the pump. There is a splice pack located near the tank that houses the ground for the pump, sending unit, and a few other circuits. This has been found to be a problem on a few other member's trucks and is worth looking into.

 

Ok will check the ground and if it looks good I will get a fuel pump assembly and see what it does. I'll post what happens.

Thanks, Andy

 

Thanks for the help guys. I replaced the fuel pump and the engine started right up. One problem though. The truck now have an evaporative system low pressure trouble code setting. This was not an issue before I changed the pump and that is all I did to the truck. What could be the cause of this? I did notice that all three lines hooking up to the fuel pump were the same size and direction. Could I have mixed up two of them? Return and the other non pressure line?

Thanks, Andy