1997 blazer 4x4 roughly 130000 no major issues till now ran fine when I parked it at night next morning bad misfire with a backfire out the exhaust

I have replaced ign coil dist cap & rotor spark plugs

Tested ( using troubleshootmyvehicle.com)
Ign module. Crank sensor. Cam Sensor. Maf sensor. Map sensor. Egr valve
Fuel pressure at 60 psi
All cylinders getting fuel
Updated fuel spyder at 80000
Full tune up at 80000
Lower intake manifold gaskets at 80000
Fuel filter recently

Any ideas?

 

Plugs, cap, & rotor need to be AC Delco.


60psi fuel pressure at the service port is 25% of the data necessary to diagnose the fuel delivery system Here is how to check all of it. Post all 4 readings: https://blazerforum.com/forum/2nd-ge...eakdown-88305/


Just curious, how do you know all cylinders are getting fuel?


How old are the plug wires?


Is the check engine light on? Are there any codes in memory? If so, what are they?

 

All parts are ac delco
62 psi while running all spark plugs smelled like gas when I changed them
The wires have around 30000 on them they ohmed ok and I tried spraying water on them at night I didn't see any jumping
The only code I'm running is p0300 with a flashing engine light

 

P0300, (random multiple cylinder misfire) is almost always caused by a fault in secondary ignition, (Plugs, wires, cap, rotor, ignition coil). Could also include a worn distributor gear and or bushings, and incorrect camshaft retard adjustment. With the engine at normal operating temperature, (~200F): Check fuel trims, both long and short term, at idle and at 2K RPM, and post them, (8 different numbers). This will tell if the misfire is rich or lean, which will eliminate a bunch of possibilities and point us in the right direction to look for the problem.


62psi while running is too high and it will cause a rich fuel mixture. The link in post #2 explains why engine running fuel pressure is basically a meaningless test. All pressure readings, and leakdown tests, are made with the fuel pump running, and the engine off.

 

I did leakdown test today
Pump on engine off 61psi
Pump off 55 psi
Held for 20 min till I took the gauge off
I have access to a craftsman scanner to see live data but I honestly don't really know what to look for

 

Ok I got the fuel trims but the numbers jumped around quite a bit so it was a little difficult

At idle engine warmed up
Stft b1 0_-4
Ltft b1 6.2_16
Stft b2 0_1.5
Ltft b2 1.5_0
At 2 k rpm
Stft b1 0_17.9
Ltft b1 6.2_25
Stft b2 0_-.7
Ltft b2 .7

 

In a nutshell, it's running lean at 2K rpm on bank 1, (driver side). It's not real serious, but none the less, it's lean, (too much air or not enough fuel). That lets out secondary ignition as a possible problem, as failures there would point to the fuel not being burned. We're looking for things that can cause lean mixture such as: A vacuum leak, but mainly on bank 1, which is where PCV, HVAC, and the brake booster mainly draw from. Could also be a restricted injector, or a lazy oxygen sensor, or a switching problem in the sensor. A good visual inspection is in order to look for vacuum leaks. Possibly the rubber PCV elbow or the O ring in the fitting in the intake manifold, or even a broken PCV valve. Injector balance test would confirm or deny a restricted injector, but that requires a capable, (high dollar) scan tool. The "work around" is dump some Chevron Techron in the fuel tank and drive it, might take a couple of tanks though.

 

Ok I checked vacuum hoses already they are good as far as I can tell pcv is pretty new and seems to be functional I replaced the fuel spyder with the updated injector per cylinder setup about 50000 ago but all the 02 sensors are original so maybe I should look at that? And it's not really drivable the way it is so I don't think the fuel treatment will work I do normally use regane full system cleaner at each oil change

 

Oxygen sensor reaction time and switching speed naturally slows down during its life. Output voltage also drops, which the PCM interprets as a lean mixture. It has no idea if the sensors are reporting correctly, so it has to assume they are correct. It will richen the mixture trying to achieve the proper mixture, and that's when the problems start. At 100K miles, the sensors have slowed enough to make a substantial impact on engine performance. The sensors are the primary source that the PCM uses for keeping the fuel mixture at 14.7:1, (perfect mixture). The faster they can react to changes in fuel mixture, the better the performance and fuel mileage will be. At 130K miles, they are costing you money in fuel, guaranteed. Definitely worth replacing them, and rechecking fuel trims. AC Delco and Denso have the fastest reaction times of any sensor on the market. Denso might be a hair faster, and AC Delco last a tad bit longer, either one is a good choice.


Absolutely be worthwhile to do the remaining steps in testing the fuel delivery system too. The pressure you see at the service port, is either regulated pressure, or fuel pump maximum output pressure, whichever is lower. For example, if your pump can only build 61psi, it would be normal to assume fuel pressure is OK. In reality, you have no idea what regulated pressure actually is, until you know what the pump can build. 61psi regulated is fine, but if that's all your pump can build, there's a serious problem somewhere


You mentioned replacing the crank and cam sensors: If the crankshaft position sensor and or timing cover are moved, removed, replaced, or disturbed in any way, a crankshaft position sensor relearn MUST be performed. If not done, the engine will start and run, but ignition and injector timing will be incorrect. The relearn requires a scan tool capable of initiating the relearn.


The camshaft position sensor is similar. If the distributor, and or camshaft position sensor are moved, removed, replaced or disturbed, camshaft retard needs to be checked and adjusted if necessary. If it is not within spec, cylinder misfire detection and accurate cylinder identification is not possible. It can also cause crossfire inside the distributor cap, (misfire, backfire). The data is viewed on a capable scan tool, listed as camshaft retard, or CMP retard, depending on the scan tool software. The adjustment is made by slightly rotating the distributor.

 

Thank you for the very detailed response it makes a lot of sense and I will definitely be replacing both upstream 02 sensors ( the other two will not effect performance correct?)
The other fuel pressure test you want me to do is the max psi test straight off the filter?
And no I have not replaced either cam or crank sensor only tested them
But when I did the LIM gaskets I did remove the distributor when I put it back I had no engine light and it ran great so I assumed it was good. Shoould I have that checked after I get this problem figured out? I just didn't want to waste the money if I didn't need to.