Hi Guys,

Truck is 99' Blazer 4x4 4dr LT

Problem: The other morning i went to start my truck and it would start then run for about a second and turn off. It also would sometime start but run at 200rpm and rough then would go to regular idle at 800 rpm smoothly. Finally after 6 attempts it would start fine like normal and run smooth. Today when i was doing pressure testing it is starting every time and running smooth acting like it never had a problem, but i know something cant be right.

Did some fuel pressure tests according to tech article:

Results are as follows:

Turn key to on and i can hear fuel pump whine for 2 seconds. This is always happening even when i was having starting issues. So i think i have good connection and pump is activating.

Hooked up fuel pressure gauge to schrader test port. Turned key to on with engine off. For the 2 seconds or so the pump is on and audible whine the gauge reads approx 61-62psi. Once this 2 seconds is over the pump stops and the pressure goes to approx 52-54psi. IF i let it sit for a bit the pressure slowly goes down.

Turned the key off, the pump runs for 2 second as it should and pressure goes to 62psi. Then falls back to same 52-54psi when the pump turns off.

Now i turned the engine on and let it warm up to operating temp. Pressure gauge reads while idleing approx 54psi.

When i stomp on the gas and rev the engine up the pressure gauge goes from 54psi up to 62 as the rpm goes from idle to ~3000. Then as it immediately drops from 3000-idle the pressure drops to 50psi, then comes back to 54psi at idle.

If someone can please give me their opinion that would be great.

I am prepared to order new fuel pressure regulator or pump, but just trying to determine if i should need to do pump or not. I am definitely regardless going to replacing the fuel filter.

Thanks, Paul

 

thats a good one i would check w meter during these conditions for battery voltage, alternator or batt may be weak too. either pump or regulator can do this but im gona imo lean towards the regulator first because when u hit the throttle it looks like pump volume is there from what u say, and i dont know ur money situation but if i was gona pull the plenum i would put the update mpi unit in. regs like 65 and new unit is like 180 on amazon. ull save alota headaches in the long run. go delphi or delco NEW update unit makes a big difference. get a gaske set too and filters due every 25 k remember, letting the filter go is what takes out the pumps early on gm's imo. good luck

 

Battery and alternator are good. I have solid 12V and 14V.

What is a quick way to determine if pump is definitely ok or even not OK?


Would I disconnect the fuel filter IN line from the fuel pump and hook up to pressure gauge to this disconnected line from the pump and turn ign to ON and see if pressure goes to 60-66 and stays? Will the pump turning off after 2 seconds affect this? I would need a constant pump ON signal?

Thanks, Paul

 

96+ Fuel System Diagnosis

 

I tried reading this entire document and performing test but i am a bit confused by the two tools used to test the fuel pressure at the fuel filter and their application. I also am confused as to my ability to test if i dont have a scan tool to run pump continuously, vs just the 2 secs when i cycle ignition. Also i will need to fab up an adapter to plug pressure gauge to fuel line from pump, i can handle this, but just want to confirm if it as simple as unhooking IN line from pump at filter and hooking/adapting gauge up to this now disconnected line and turning ign to ON and seeing pressure. And if i see pressure what should it be (60-66psi or 66psi+?) and should it hold it after pump deactivates.

I was hoping someone could sum up, baring my results already, in one paragraph what i should do to definitely solve the question of New pump or No new pump. I am sorry just a little confused from the troubleshooting guide.

Thanks again, Paul

 

Paul, Welcome to the forum!
Your fuel pressure readings are fine, leak down might be a tad bit excessive but nothing to worry about if that's all it is, (technically it shouldn't drop below 55psi after 5 minutes). If it drops any lower I would suspect a leaky check valve in the fuel pump, very common problem. GM does not publish any "engine running" fuel pressure specs for this engine, but from what you've posted, the regulator is working flawlessly. Due to the intermittent issue, it might not be a bad idea to leave the pressure tester connected and see what fuel pressure does when the problem is present. That'll at least tell if it's fuel delivery related.

 

Thanks captain,

I appreciate your input, and i will take your advice. As i borrowed the fuel pressure gauge to do the testing today i will go out and buy one. These are handy to have in a toolbox when needed anyways. I figure i definitely have some intermittent issue, however i wont jump and spend money where it isnt needed until i know more. So i will hook the gauge up and leave it hooked up. In the coming days, weeks, maybe even months til it happens again. When it does i will then post my results immediately from the already hooked up gauge, see what is really happening.


Thanks, Paul

 

Question, does it only happen on cold DAMP mornings? Could it be spark leak from your wires? bad connection at the coil? worn distributor cap getting condensation in the contacts?

When was the last tune up? What brand tuneup items were used?

Granted MOST of the time you would notice other signs that your ignition system had problems when it WASNT first start of the day, but you may be catching something early.

 

HI Mike,

I bought the truck about a year ago. Havn't done any maintenance on it yet personally. Has 162,000 kms on it.

I would say that the other morning when i had the problems where it would start, then turn off, start then turn off, etc was colder than yesterday and today which it seems to be starting fine.

I have installed the fuel pressure gauge and will leave it installed. IF/when it happens again i will quickly be able to pop the hood, position the gauge against the windshield and watch what is going on with the fuel pressure.

For now i will have to wait and see. Perhaps it would be a good idea for me to replace plugs, wires, fuel filter, etc. as a bit of a preventative tune up.

Thanks, Paul

 

Yeah I'd recommend doing a full tuneup. Use only ACDelco tune up items as others tend to cause more problems then they solve. Great idea with the fuel pressure gauge that should help you diagnose.