2000 4.3 Blazer misfire cannot diagnose
 

Hello all. My first post.

I have a 2000 Chevy Blazer with 170k miles on it.

It started misfiring recently which caused the SES light to come on. I tried nearly everything and still cannot diagnose the problem.

I get the following codes:

P0301 - most of the time

P0300 - some of the time in lieu of P0301

P0171 and P0174 - some of the time

P0302 - came on once

Obviously, the codes change without any rhyme or reason.

The car runs but shakes a lot around 1800 RPM. It seems that it gets smoother when it's over 2000 RPM.

Gas mileage is around 9MPG.

Some days it runs better than the others.

When under load (usually over 3000RPM), the SES flashes.

The fuel pressure is good, pressure remains the same after shutting the car off as supposed to.

At idle fuel trims are over the roof. STFT is frozen at 50 and LTFT is at 25.

There are no vacuum leaks.

So far I changed spark plugs, wires, distributor cap, rotor. Still the same.

What bothers me the most is the inconsistency with which it switches from P0301 (Cyl#1 misfire) to P0300 (Multiple Cyl misfire).

Anybody had the same symptoms? What could it be?

had almost same prob it was the cam sensor under the distributor cap ended up replacing the whole dist no more misfire

Probably not related but... on my Grand Prix the SES starts flashing around 4000rpm because of the presence of cylinder fire occurs when it is not expected. This happens when I run below 91 octane fuel.
Also, a Dodge Stratus has a misfire on 3&6. The SES will begin flashing while going down the interstate about 3000rpm. Best I can guess is that I collapsed a few lash adjusters(lifters) when I did the timing belt. I'll be pulling them in the spring.

I just wanted to mention this because it could be a lean condition. Figuring out what is causing it is the tough part though.

1. What is fuel pressure when the pump is running and the engine off?

2. What is the rpessure 10 minutes after the pump shuts off?

3. Post all 4 fuel trims at idle with the engine at operating temperature:

LTFT bank 1 & bank 2, and STFT bank 1 and bank 2

4. Post all 4 fuel trims at 2000 rpm with the engine at operating temperature:

LTFT bank 1 & bank 2, and STFT bank 1 and bank 2

Make sure you indicate positive or negative trim numbers.

5. What brand and part number were the plugs, cap & rotor?

6. Did you replace the plug wires?

Yeah definitely need to know if those fuel trims are + or -. If + you've got yourself a GIGANTIC air leak somewhere ... more than likely the intake manifold gaskets (lower) ... but could be anything from a failed to disconnected vac line too. If negative trims, it could be a leaky injector or spider, or a failed regulator. The 2 most common failures on these engines are the spider and lower intake manifold gaskets. You can spray carb or brake cleaner around where the manifold meets the head, and note any RPM changes - if there's a big change, then you found the leak.

This, right here, is why I LOVE carburetors. ;)

Another thing happened today. I got P0147 (O2 Sensor heater circuit malfunction on bank 1 sensor 3) for the first time. I didn;t come back after I cleared it off.

1. Somewhere around 55psi. Pressure does not drop after the engine shuts off.
2.I didn't check that when I had the gauge but I assume that it holds. I know that it doesn't drop much lower than 50 psi after 5 mins.
3.Idle:

LTFT bank 1 is 25 bank 2 is 25

STFT bank 1 is 35 bank 2 is around 8

4.2000 RPM:

LTFT bank 1 is 9 bank 2 is 9

STFT bank 1 is around 25 bank 2 is low around 3

All fuel trims are positive.

5.Generic brand

6. Yes

As mentioned, positive fuel trims indicate a lean mixture. The PCM is trying to richen it by leaving the injectors open longer. A lean mixture is caused by either too much air or not enough fuel. Too much air is usually due to a vacuum leak. Too little fuel can be caused by restricted injectors or low fuel pressure.

The PCM is calibrated to inject a given amount of fuel in a given amount of time, with fuel pressure between 60psi and 66psi. If the pressure is low, not enough fuel will be injected. The PCM does not monitor fuel pressure, so it "assumes" it is correct. In an effort to richen the mixture, the PCM leaves the injectors open longer, hence the "positive" trim. Your reading of 55psi is too low. Needs to be a minimum of 60psi. This explains the DTC's you're getting, with the exception of the P0147.

The problem could be the fuel pressure regulator, however, if the pump can only build 55psi, and the regulator is set above that, regulated pressure will be 55psi, (maximum pump output pressure). Easy way to think about it is: regulated pressure will be the lower of regulator setting and maximum pump output pressure. Pressure needs to be checked at the fuel filter outlet to determine if the pump is capable of building proper pressure. All fuel pressure and flow must end at the tester, with no fuel allowed to the engine.

Plugs, cap & rotor may be contributing to the misfires and really should be AC Delco.

THIS, is why I LOVE computer controlled fuel injection. The onboard computer tells you exactly whats going on. As long as you understand what the data means, they're a breeze to diagnose;)

You need to test the fuel pressure with the engine off (key on engine off)

Well, not exactly. It will point you in a general direction, but never tells you precisely what the issue is. For example, a P0300 could be caused by any number of issues - none of them ignition related either. That code is my nemesis ... :icon_lol:

I had a 1 second misfire that happened about 20 to 30 times in the first 10 or so miles every other morning. I had a tune up 10k earlier (cap, rotor, wires, injector cleaning) so I didn't think that could be the cause. After a month of this, after reading other threads, I finally decided to replace cap/rotor with ACDelco brand and it immediately fixed all the misfire problems.