My 98 Suburban (251,000 miles) has been running bad the last couple of weeks and is throwing a CEL, and scanning it shows four codes:
P0420 (Bank 1 catalyst effciency), P0430 (bank 2 catalyst efficiency), P0135 (O2 sensor bank 1 sensor 1), and P0305 (cyliner #5 misfire).

Occasionally, the CEL will actually flash instead of staying on. I replaced both O2 sensors, pre-cat, as I knew at least one was probably original. Also ran a can of Seafoam thru the gas tank on Sunday and it ran a little better on the highway going to Raleigh. But Monday ran like crap again. I'm thinking two things: Injector clogged or malfunction AND/OR catalytic converters are toast. I found the universal Magnflows for close to $80 each. Also thinking about the CSFI to MFR conversion?


Any ideas on these symptoms? I've never seen a flashing CEL previously.
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the converters or exhaust may be restricted. the injectors may be dirty/worn causing excessive fuel to get to the converters.

plug wires should be checked for proper resistance. also check the fuel pressure with a guage 60-65psi,..you may have a coupe of problems.

the O2 sensors are good for 100,ooo miles usually 150,ooo if all hywy driving. going over this amount is looking for trouble and lower MPG/performance.
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Thanks for the quick reply. Plug wires, plugs, rotor, and cap were new in August, 2010. New fuel pump this past March (AC Delco). I just replaced the pre-cat O2 sensors, both of them. So the P0135 code would likely have gone away. I'm guessing all of this may be due to a poppet malfunction on the CSFI injector system, but the OBDII on these Vortec motors really can't get that far into diagnosis like the OBDII on an MFI car. Pre-cat O2's are the important ones that actually feed back into the PCM, to set air to fuel, etc. The post-cat O2's are just the monitoring ones, that tell you what sort of crap you might be putting into the atmosphere.

Thinking it may be high time to replace the old CSFI injectors with the MFI system. Amazon has the Ac-Delco one (217-3029) for $285 or the Delphi (FJ10566). Both come with the bracket and fuel o-rings. I think the Delco one may even come with some gaskets (e.g. upper intake and another??)
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with a fuel pressure guage you can determine if the injectors are leaking down. also checking the fuel regulator.

miss fires can be from low fuel pressures and restricted cat . since it comes and goes It can be fuel pressures low as the fuel pumps do fail sometimes with good pressure then drop down when they get hot or the fuel level drops.

first find the cause of the cat failure . it could be the old O2 sensors caused this. the upstreams sensors are most important. the downstream which monitor the cat performance are now too old . I doubt the are reporting whats actually occuring with the cats.
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Thanks again J Cat,
Yeah, i have a fuel pressure gauge and that's exactly why i had to change out the fuel pump this Spring - low FP pressure. Truck would not even start. Was only getting about 40-45psi, and that's barely enough to push the poppets open on the CSFI system. At that time, the regulator tested good, since the best way to test it out is to pinch off the return fuel hose and see if pressure climbs. It did not, so I ruled out the regulator back then. I put the new pump on and all was well (for a while). So, both upstream O2's are new, just replaced early this week. Truck ran well on the Interstate for a while, with the Seafoam in the tank, but the next morning, ran like crap again. So you're thinking my downstream O2's might just be bad and I might not really have anything wrong with the Cats? I had thought back in the Summer when i was occasionally seeing this problem, that my coil was bad, but although it is original, it spec'd out fine, per the service manual tests.
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If your CE light is flashing, it means your vehicle is misfiring, and if you continue to drive like this, you just create more problems.....

You have high mileage......how many times have the cats been changed?

The cat efficiency codes don't necessarily mean the post O2 sensors are bad...you have to look at the wave forms and compare them to the pre O2's, to determine if they or the cats are bad....
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Cats have never been changed, far as I know (I've only owned truck since 2007). I use TTS' DataMaster Software for engine PCM diagnostics (and TunerCat to flash PCM), but clearly, it's just for OBDI cars and won't work for the OBDII systems. So really, my only diagnostic is the simple Actron tool, which just gives actual code. I don't think a $65 per cat would be bad to pay, for new cats. Magnaflow 99004HM.
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when the vehicles acts up throw on the fuel pressure guage.

most find delphi replacement pumps work best. airtex fuel pumps rapidly fail. If you have an airtex pump that would explain some of these issues.

the distributor bearing should be checked with the miss fires.170-225,ooomi these usually get worn to the point of creating the miss fire.

If the down stream are original I would replace. too many miles on them.

If I had a O2 sensor code , I swap sensors then see if the code changes.

these sensors most times slowly stop working correctly. with an expensive diagnostic machine you can see the waveforms then you know for sure if they are working correctly.
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Thanks...Again, for the good advice here. Yeah, I know about the Airtex fuel pumps. That's why i put the AC Delco fuel pump on the Suburban. Many of the fuel related Delco parts are Delphi (even though Delphi is now independent). Doesnt work that way on everything, but many of the sending units and pumps. I checked the distributor gear (I think you meant that and not the bearing?) when I changed over my lower intake gasket and oil pressure sender last July. The gear actually looked good when I had the distributor out. I was prepared to change it, but it looked good.

I thought when the gear goes, you lose all timing, etc. and don't show misfire, because the timing jumps a tooth or two.

On your suggestion to switch out downstream O2's, BOTH are throwing codes, so really, switching may not do any good. I could replace 'em though. Just more $$.
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the distributor cam gear can be worn with that many miles. since you looked at it not that long ago that should not have worn.

I was refering to the distributor bearing. these will cause the rotor to wobble at times when first failing giving missfires.

If you swap the O2 sensors see if the codes change or stay the same this will tell you if the O2 sensors are bad. over 150,ooo mi on a O2 is too long of use. manufactures say 100,ooomi. .
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New development. No more codes for cats! All are gone, perhaps the Seafoam helped out. But the bad news: CEL still flashing and now is throwing a P0300 code, which is now a random misfire code instead of a specific cylinder like before.
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random miss fire I would check for proper fuel pressures.

also the distributor components, bearing etc.
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Updates: I just went to Boone, NC to check on the truck (it is my daughter's and I rarely get to look it over for long):

1. Fuel pressure tested fine. 58-60psi steady
2. Ignition coil tested fine.
3. Friday evening, the truck just quit - I believe my daughter ran it out of gas, but she got it started back up, after putting more gas in it. (This is unrelated to problem).

A couple more things - my daughter said she noticed a 'pop' when starting up the truck. Wondering if that is a clogged fuel injector poppet trying to open? I also noticed a smell of raw fuel at the exhaust. Thinking one or more of the injectors on the old CSFL system has totally crapped out. Isn't it true that if injectors are the problem, there will be random and multiple misfires? Thinking I need to get the Delphi FJ10566 or Delco 217-3029....to convert the CSFI to MFI?
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If your daughter operates this vehicle with little or NO fuel in the tank the fuel system gets destroyed.

fuel pump overheats, injectors get damaged with water[condesation] creating a growing mass of contaminates generated by the water. the fuel level must be at all times above the 1/4 mark...

during certain times of the year with large temp swings day/night water rapidly accumulates in a fuel tank with little fuel.

back fire on cold start good chance lots of contaminated fuel..................
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Actually, she has never let the truck run out of gas until Friday. I agree on the FP overheat. the fuel, believe it or not, is what keeps the FP running cooler. The sound she's hearing is definitely not a backfire. I ran another bottle of the Chevron/Techron fuel system cleaner in the truck, but it didn't seem to help this time, like the Seafoam did a few weeks back. This misfire thing, however, I can't really blame on low fuel level and water. When she was home over the Summer and this problem was just starting to surface, fuel level was fine. It actually started happening during her Spring semester of 2011, back in January. But it was erratic and wasn't constant. It has now grown consistently bad. so much so that misfire codes are thrown.
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