**FIXED** 2001 Suburban 5.3 ..p0300 ..random misfire on cylinders 7 and 8 ..

Hey guys ..brand new here ..and have seen a few of SDs YouTube vids ..love the vids and this site ..thanks in advance for any assistance on muhbaby ..I do have an Autel scanner ..and am impressed with its flexibility ..and am using fuel trims and O2s to get this figured out but I’m kinda at an impasse ..before I acquired my scanner I’ve managed to throw parts and wasted time at this issue for a good while ..so am thinking a more clinical/professional approach is needed ..so here’s the deal ..my fuel trims are good ..just last night ..reading normal ..STs fluctuating 1-3 ..LTs around 0-3 ..O2s (both banks/upstream) are chattering from 200-800mV ..the down streams are pretty much flatlined at 700-800mV ..correct me if I’m wrong ..but from all of the research I’ve done these are decent readings ..that being said ..parts changed before scanner have been AC plugs ..AC wires ..intake plenum gasket ..cleaned the throttle body and MAF sensor assay ..checked the EGR ..had a 48 psi reading on the fuel pump so changed that ..checked the fuel pressure reg ..(not leaking) ..no vacuum leaks ..clean air filter ..blah blah blah ..(frustrating) ..
..so ..here’s what happens ..when I say RANDOM ..it truly is random ..my issue comes and goes ..example ..it was acting exceptionally bad the other day so I broke in to a new plug wire set and changed 7 and 8 ..started up ..scanned ..and perfect ..no misses on idle ..ran great ..idled for 20-30 minutes ..(I thought ..really? ..it was THAT simple ..after all I’ve done?) ..so ..I took it for a test drive ..ran it hard for 5-8 miles ..with the scanner still hooked up and looking at misfire graph ..running great ..finally came to a stop light ..slowed ..stopped ..waited ..20 seconds later ..started missing again ..on both cylinders at the same time ..grrrr ..so ..I raced it on back to the garage ..it’s weird ..kind of at a loss ..I’ve actually got 2 coils coming in on Friday and plan on installing them ..but already am thinking it won’t work as why would BOTH cylinders start missing AT THE SAME TIME?? ..something external only common to those two cylinders? ..geez guys I’m at a loss ..anyway ..let me be clear ..this ONLY MISSES AT IDLE ..running at anything above idle ..always runs clean ..no misses ..the scanner also verify this ..anyway guys let me know your thoughts ..I’ll certainly be posting my results here as well ..thanks again in advance of any direction on this ..BE WELL! ..Mark

From the just of it I don't think you were in closed loop yet both upstream and downstream O2s should move up and down in voltage readings. If it is two cylinders at same time missing. You will need to smoke test the intake when engine is warm. Did you torque bolts with torque wrench for intake plenum and torque in specified sequence? check map sensor seal on back side of intake when smoking intake

It would be of help if you could provide more detailed information about the conditions when the misfire happens, .......

So far I take it you have an intermittent miss, at idle only. Misfire at idle only often is due to a valve issue and if it is intermittent it might be a dirty/carbonized/pitted valve. Put a vacuum gauge on it if you have one. Otherwise try to graph the MAP at idle or even better put a scope or a really fast voltmeter in min/max function on the sensor.

Did you look at the misfire counters live data to determine tha 2 cylinders are misfiring?

With regards to the O2 voltages: They are perfect for an engine that runs without a misfire, nothing to worry about.

It was in closed loop ..I don’t have a smoke machine ..although I’m thinking I’ll either get one or take it to someone who has one ..I’ll try the water test ..up around the intake ..(although I did replace the gasket before ) ..I’ll check the MAP sensor and seal ..I’ll post ..thanks

Well ..I’m not sure what additional info I can provide ..but I’ll attempt ..it’s most evident when the vehicle gets to operating temp ..I explained that I idled it at least 20 minutes (ran perfect ..no misses) after replacing the plug wires in 7 and 8 ..and also explained while I had my scanner on ..looking at misfire graph ..that I ran it hard on a test drive and only started missing again ..simultaneously on 7 and 8 when I came to a stop light ..someone else has mentioned the MAP sensor ..I’ll definitely take a look there ..and will educate myself on its function ..thanks for your input ..good stuff ..I’ll post further ..

While a agree with Juergen about idle misfires being a good indicator of a valve problem, I would add that EGR is something you should be thinking about. It being an older vehicle I wouldn’t trust the misfire counter down to the cylinder 100%.

When you say checked the EGR how did you check it?

When is the last time the valves have been adjusted? Too much valve lash will open the valves too late and close early.

FIXED! ..Ok well ..let me first say that I appreciate the input ..you guys are awesome ..
..so anyway ..let me respond to the last post by saying that I had previously checked the EGR by toggling (open/closing it through my Autel) ..and it was responding as the idle would change significantly as I played with the parameters allowed ..so I dismissed that as the culprit ..as far as the valves are concerned ..I had replaced the valve cover gaskets a some months back ..and wasn’t really looking at valve adjustment ..or considering at the time ..but ..can these rockers BE adjusted? ..with the classic feeler gauge from years ago? ..I’m thinking no ..but could be mistaken ..but in fairness to the post ..the engine has 202k miles ..and this definitely would have been a possible cause ..
..as far as the smoke test ..from a previous poster ..I do not have a smoke test tool ..(wish I did as I’ve seen vids that use them and they sure do find the issue pretty quick) ..but checked for intake/vacuum leaks previously ..and did AGAIN just to reconfirm ..all good ..
So I planned on following through with the coils and then next step ..quite honestly ..I planned on doing a top end rebuild ..figured might as well as it would reveal a lot about valve conditions and figured no biggy anyway with the miles (202k) ..
..so when the coils arrived late Thursday ..I popped them in ..started up ..and ..well ..BOOM ..idled clear/smooth ..took for a quick test drive ..all good ..next morning ..started fine ..all good ..ran it around town ..and up to hwy speeds to set my IMs ..I’m thinking it was ..over a period of time ..a combination of things ..because this vehicle was not maintained very well at all ..that being said ..I started Sat morning ..ran it around a bit ..put the scanner on it ..IMs are all set ..(BTW ..the ‘check engine light cleared itself ..organically) ..so I took on down to muhsmog guy ..passed ..and now ima happy boy ..I’ve attached a pic of ‘Bob the Burb’ ..I’ll keep him til I pass ..still needs some stuff done to it ..minor things ..a few leaks to address ..but over all ..he’s got good bones for his age ..(like me!) ..thanks again guys ..M

Glad you got it fixed.

Although it's not uncommon for coils to fail when hot it's a bit of a surprise to see them fail at idle only and not in a high load/demand situation.

Anyhow, one more happy camper...

That's a fine looking 'burb