00 Suburban 5.3L P0300 Random Misfire

I know you guys get atleast one of these every week, but humor me. It started like so many other posts; accelerating uphill on a cold, damp day it started missing, wouldn't make the downshift and the SEL light started blinking. (By the way, the SEL had been on since the dealer took care of the Fuel Module recall a couple of weeks ago.) Running flat on the road, it returned to normal, with the SEL illuminated.

A few days later, the same thing happened again. This time, it doesn't return to normal. It begins to miss so badly that I barely make it home at 50 MPH maximum speed. Next I read all the posts I can and purchased some supplies.

Yesterday, I scanned the OBD II terminal and came up with both the P0300 and the P0420 (left bank catalyst). So, I pulled the AC Platinum plugs, which looked okay overall, though the left bank might have been ever so slightly darker than the left. Today I replaced them with AC Iridiums. I removed and inspected the Delphi/Packard plug wires. They looked fine and all measured around 4250 to 4400 ohms. I cannot see anything unusual about the coils themselves, or the wiring leading to them. In the dark, I cannot see any visible arcing anywhere on the engine.

I replaced the fuel filter and checked the fuel pressure. Key on: jumps to 58 psi, drops a few pounds then maintains 56 psi steady. After starting the engine, steady 52 psi. Engine vacuum measures 15" initially, then reaches a steady 17" within one minute. Thinking about a possible clogged catalytic converter, I checked the vacuum at 1000 rpm and roughly 2500 rpm; only about 3" apart. I don't think that is enough to indicate an exhaust restriction??? After a reset, only the code P0300 returned.

I removed and inspected the air filter (clean), the MAF sensor and the EGR valve, and the PCV valve -all of which looked good. I am thinking it might be an injector problem or bad gas, as long as those fuel pressure readings are okay. Though some other posts have identified the Cam Position Sensor as the culprit, I'm not sure of how this is diagnosed. I am generally not in favor of replacing parts just to eliminate them as the cause of the problem.

I would appreciate your opinions.

Follow up: cleared the trouble code and added a bottle of Chevron fuel injector cleaner to the tank. I started the engine cold (24 deg F) and it idled smooth, without missing. I let it warm up and the misfire did not reappear at idle. I took it on the road for a test drive and within 1/4 mile: the random misfire, along with the SES light, appeared (blinking). Unlike the misfire going up hill/under load, this misfire doesn't go away once it begins....

Now, because this seems to have occured when (I assume) the engine went from closed loop to open loop operation, I am focusing on electronic (spark control) issues. I will begin by checking the cam position sensor tonight, as many posts seem to point it as the cause of similar random misfire issues.

If the cause is, in fact, the cam pos. sensor, why doesn't it register one of the many CPS trouble codes??? Apparently it can send erroneous signals that don't trip the SES light?

I'll post more later. Thanks.

While I have never had this problem with my suburban, I have had very similar problems with a 95 cutlass and it was the coil packs (2 went out within a month).

I have the exact same problem with my 97 tahoe and I always get the P0300 code. The chevy service shop finally figured out it was the fuel injectors. After this fix I think I gained about 50 hp also.

I took a chance two days ago, and replaced the cam position sensor. Many other posts have pointed to this as the cause of the mysterious "Random Misfire". It's fairly hard to get at, and never having done one before, I removed the intake manifold to get at the rear of the engine, where it is located.

Sadly, this did not solve the problem. I reassembled the intake and injectors, etcetera, then started the engine. It ran very well at idle. No missing whatsoever. Hopeful, I took it on a test drive. Once again, as soon as I got out on a main road, it started missing under acceleration. And once again, once the missing began, it did not stop. It continued missing under all conditions, including idle.

I am convinced that the problem is electronic, based upon the perfect idle when cold. If the problem were mechanical, I would expect it to be present at all times. I also think that the problem is occuring when the engine goes from open loop operation to closed loop. (I think I got that backwards in a previous post.)

If the problem is with the fuel injectors, I will know soon. I purchased a OBD II/VPW protocol connector to monitor various engine functions. It works with software which is widely available on the www, and loaded into a laptop computer. Though not exactly what they use in repair shops, it does provided hundreds of times the information that a basic OBD II code scanner will provide. It does monitor fuel trim/pressure/and O2 sensors, among other things.

I will post back again when I learn more.

The cats are bad.Ran into a problem like this and the cats were bad after replacing a 5.3 that went bad.The catalytic converters will kill a 5.3 if they go bad what I am hearing.Hom many miles on it?If it is under 80,000 miles,it is covered under warrenty by the dealer and you might want to check into this.Most dealers stock the catalytic converters because there are problems with them.

"Engine vacuum measures 15" initially, then reaches a steady 17" within one minute. Thinking about a possible clogged catalytic converter, I checked the vacuum at 1000 rpm and roughly 2500 rpm; only about 3" apart. I don't think that is enough to indicate an exhaust restriction??? After a reset, only the code P0300 returned."...

(135,000 miles) With only 3" difference in vacuum between 1000 and 2500 RPM, I was hoping I had eliminatated the CATS as the source. Once I figure this OBD II software out, I will be able to monitor the O2 sensors to see what's going on back there. I will post more then.

Thanks for the input.

My neighbor did have that problem,luckily under warrenty under 80,000 miles to the dealership that he once worked at.It was the catalytic converters and the plugs were changed at the same time.

A blinking MIL light specifically identifies a fault with your catalytic converter. If your vehicle has exceeded the milage under which your cat is covered, you may want to try tremporarily unbolting it to see if that helps. You may have other problems, but you may want to try eliminating this first.

I confirmed at-least one diagnosis today; the restricted catalytic converter. I set up a back-pressure tester on the passenger side, just to see what it measured. Not even one pound. At 2000 RPM it just fluctuated around 0-1. The driver's side measured 2-3 at idle, and went up to around 6-9 @ 2000 RPM! This clearly explains the slightly fouled plugs and the catalyst efficiency code, in addition to the random misfire.

Since my laptop is giving me fits, I haven't been able to monitor the engine emissions functions yet. I removed both driver's side oxygen sensors and tested them off the car. I connected a DC volt meter to the output lead and applied heat to the sensors. Both produced approximately .9 V when heated and dropped to less than .1 V when the heat was removed. There were no voltage fluctuations during two minutes of heating.

So, my question at this point: Did the converter gradually plug up and cause the misfire/plugs fouling -or- did something else cause the misfire which resulted in a clogged cat/converter? Also; is there anything that can clear a clogged cat, or does it have to be replaced?

Thanks...

Finally figured it out...Driver's side CAT was clogged. The front O/2 sensor on that side had been wearing out -slowly. I talked to a tech at a local dealer who informed me that they see this type of thing (random misfire w/or w/out the catalyst efficiency code) often. The vehicle can check out okay during a diagnostic scan. But, when they have replaced the front O/2 sensor just to see... that takes care of the problem.

Anyway there is an excellent article on evaluating O/2 sensors at this link: www.troublecodes.net Go to the BAT Auto Resource Center, then the Technical Articles section and check out the one on Catalyst Efficiency Failures.

If you run into this problem and have ruled everything else out...replace the upstream O/2 sensor.