Maybe someone can help diagnose this. 2000 Blazer 4x4 4.3l. I have owned it for about 6 months. Stock engine 138k. I am logging KR and miss fires at various loads/rpms even idle, but not enough to throw a cel. Seems completely random to me. I want to dig into the engine calibration a bit more, but this is holding me up.

My hunch is fuel injector, but I don't want to just throw one on for ****s and giggles because they are a couple hundred bucks.

General maintenance is good. Few things I have replaced are the plugs, wires, cap, rotor. EGR is operating properly. Distributor has very little play. Fuel pressure is 59psi at key on and holds steady. Tried high octane and fuel injector cleaner for a couple tanks and it made no difference. Anything else I should look into?

 

Well even at 59 thats a little low. Try checking max pump outlet pressure at the outlet side of the fuel pump.that will tell you if the pump is making enough constant pressure and or if the check valve is working. Should be around 78 to 100 psi from the pump. connect gauge directly to the fuel filter make sure no gas gets to engine.

 

Ok. Checked twice at the filter. Pressure was at 90psi and leaked down to 75 in about 10 minutes. Then 93 to 78 in 10 minutes. Bleeds off 10psi in about 5 minutes then slows down as pressure drops.

What is an acceptable leak down?

My pump has been whining for about 4 months, like many other people on the forum so I didn't bother replacing it.

 

If there are no DTC's being set, the cause of the misfires is not going to be obvious. It will require some diagnostics on a scan tool capable of accessing streaming data. Fuel trims, oxygen sensor data, camshaft retard, misfire counts, etc, will all need to be checked and analyzed. Here are some possible causes:

Fuel delivery is critical on this engine. This thread explains how to test it: https://blazerforum.com/forum/2nd-ge...eakdown-88305/

Your max output and leakdown at the filter is within specs. This means the pump is able to provide sufficient pressure, and hold it. Leakdown at the service port is fine, indicating there are no leaks in the plenum. Key ON, engine OFF, pump running, (regulated fuel pressure) is low. Along with the other fuel delivery data, this means the regulator is set too low. 1psi below spec might not sound like much, and it's not, but low fuel pressure will cause a lean fuel mixture possibly resulting in random lean misfires.

Correct spark plugs, plug wires, and cap & rotor in good condition, is a must. These engines are real fussy when it comes to secondary ignition parts, using other than AC Delco parts, usually results in misfires. Some can be subtle, others not so much.

Crankshaft position sensor relearn data and camshaft retard must be correct. If the crank sensor has been disturbed, the relearn must be performed. If the distributor has been disturbed, camshaft retard must be checked/adjusted. Both procedures are done with a capable scan tool, and the el cheapo $500 jobbies won't do it.

The upstream oxygen sensors must be good. They must report accurately, the response time must be fast enough, and they must be capable of switching, (lean to rich, & rich to lean) at an acceptable rate. As they age, all of these things deteriorate. AC Delco and Denso oxygen sensors are the fastest sensors available, avoid Bosch.

 

OK. Thanks for the technical help. I did buy all AC Delco parts. I can monitor the O2 sensors with hptuners and they appear to be switching properly and my fuel trims change accordingly.

I am going to test the pressure again at the plenum this weekend to see if it is for sure on the low side. If so I will just upgrade the injector(old style) so I can rule out fuel delivery issues.

 

Looks like I have one other issue I will clear up first. LMK if this is 100% right. Quick search says 195 degree thermostat operating temp?

I was looking at my logs and I always log when the engine is up to temp(or at least I thought it was up to operating temp). I idle/cruise at 187-189. Only after a WOT pull do I see 195. Then right back down to 180s. Won't crack 200 degrees which it should with a 195 stat. It appears the original owner installed a lower temperature thermostat of ~180-5.

 

If/when you upgrade the injector assembly, check fuel pressure and leakdown afterwards to make sure it's correct. Just cuz it's new, doesn't mean it's good

You are correct on the stat, needs to be 195F. Normal operating temp usually ends up ~198F - 202F. Now, if it's freezing *** cold out, you might not see that temp Shouldn't notice temp fluctuate on WOT though. I suspect you're correct on a colder stat, or it's not sealing shut.

 

Got my stat in and it is operating properly.

Tested at the service port again and key on and read 58psi. LMK if you think I could get away with just a fuel pressure regulator since we are talking $40 vs $300 for inj assembly. I notice most say they dont go bad, but I don't get what would cause low pressure at the service port otherwise. Miss fires are all cylinders so that leads me to believe the injector is operating properly, it just has low pressure.

 

You need to check fuel pressure and leakdown at the service port AND at the fuel filter outlet. Testing at the service port shows regulated fuel pressure or fuel pump maximum output pressure, whichever is LOWER. 58psi regulated will cause problems, but if maximum pump output is only 58psi, that's a major problem. The link in post #4 explains how to check at both locations. Post all 4 pressure readings, they will tell us where the problem is.

 

At filter pressure/leakdown(2 tests):
90 to 75 in 10min
93 to 78 in 10min

At service port:
59 to 55 in 10min