98 suburban-check engine rough idle-sudden

Discussion in 'GM' started by phoebeisis, Oct 26, 2015.

  1. phoebeisis

    phoebeisis Well-Known Member

    Sure am glad I am a member here.
    I get much more useful 5.7 suburban info here-than on the GM truck forum
    Makes no sense-but there it is

    Anyway- on way to sub. today
    6:30 am
    Check engine flasher-actually a long phrase-service engine soon blah blah-
    comes on
    AND the idle-off idle-is suddenly ROUGH
    on the edge of stalling ROUGH
    Unfortunately I am now On the W bank of the Mississippi
    so at 2:40 pm I will have to gain 250 feet as I go over the Mississippi
    WHAT is most likely?
    No "funny noises" other than the "I'm on the edge of stalling shuddering"
    This was a NO WARNING-just all of sudden-
    It did happen JUST after I had glided down-engine on-the bridge
    Not sure if the "gas fume recovery system" can become tweaked because of that
    or the PCV or EGR or
    Anyway best guesses??-SERVICE ENGINE ROUGH IDLE\
    229,400 miles the PCV and everything else-is at least 9 years old(meant to replace-forgot)
    PS I hope it isn't the fuel pump-when I get a chance-I will strap on the SG-buzz it around the parking lot-hope the fuel flow looks "OK"-
    I quickly-in dark and rushing to work-checked for codes-but I might have accidentally cleared code-safe bet they will return-hope I can make it over the bridge-1 mile 250 foot climb to top-
    if I wasn't on the west bank-no problem stumbling home,but...

    PS If I can find an auto parts store-near here-I will change the PCV valve-long shot-but it is something I can actually do-and it is ANCIENT- and I won't get TOO filthy doing it.
    It was really really sudden-after the long glide downhill-in N since speed went from 48 at top to 67 at bottom-(glides in D as if in N at speeds under 40 mph- BUT ABOVE 40 or so I get engine braking)
    PPS- CODE PO 300 random misfire- Love that scan gauge best $100 (2005 or so) I ever spent-need a new one-
    I am hoping it is a stuck open PCV -probably isn't ,but hope springs eternal.
    I just checked it-rough idle and off idle-but just revving it- it seems smooth enough well off idle-and shows 2.8 GPH when I hit the gas(stopped not actually driving)
    Last edited: Oct 26, 2015
  2. RedylC94

    RedylC94 Well-Known Member

    Does it run OK at higher speeds and under load? When my Mazda occasionally had symptoms like you describe, it was usually a vacuum leak (hose off or cracked or ...), except the one time it was a leaky EGR valve.
  3. phoebeisis

    phoebeisis Well-Known Member

    Redy-thanks for the help.
    Just got home-made the 270 foot climb up the Miss. River Bridge

    Nah-it runs poorly at 50 mph -but not as poorly as idling or just off idle.
    Right off idle-it stumbles-feels like it would like to stall-but it doesn't.
    I have been distracted the last 9 years-my late wife was ill-so all maintenance-other than just oil changes-was deferred.

    I bought it in 2007-$2950-and only fixed/replaced "broken stuff"-radiator brake booster water pump 2 door handles front sway bar bracket

    Everything else-plugs dist.rotor plug wires pcv egr hoses(the heater hose 4 way manifold cracked in half-so I just D/Ced it and short circuited the coolant) was ignored

    faintly possible-from reading-that it is a "stuck open" PCV valve(doubt it-but it was sudden)
    Like you say-a vacuum leak -pretty good bet-leaning it out would/could cause a random misfire

    With any kind of luck-it will be simple-or at least cheap-to fix
    I have lots of catching up to do
    Heavy rain-so it will have to wait until Wed
    12.8 mpg on the trip home-normal in the rain -and afternoon traffic would be 15-15.8-mpg
  4. RedylC94

    RedylC94 Well-Known Member

    A vacuum leak (including a stuck-open PCV valve) should be a lot less obvious under load than at idle.

    Your "heavy rain" might be a significant clue, if it has revealed a fault in the ignition system, for example a bad distributor cap, which tends to show up in wet conditions---assuming it has a conventional distributor.

    Stay dry!
  5. phoebeisis

    phoebeisis Well-Known Member

    Right-it hadn't rained much at all last 6 weeks
    But the sky has opened up-8.6" yesterday-and constant rain today.
    This am was the first time in months when it was driven in the rain.

    It is less at 50 mph-much less-the miss is much less noticeable
    but it is still "off" especially when asking for any acceleration at 45-50 mph

    Bad cap-or another suggestion-cracked burned rotor-seem good too-thanks

    It is a " traditional cap" and rotor-but doesn't look ONE BIT like a "normal old time" cap or rotor-
    located inconveniently too-waaay in the back of the engine bay-buried behind the Intake manifold and FI- miserable location

    I swear I could smell a faint "electrical fire wire insulation odor" but it quickly disappeared-and the water spray might have spread the underhood gunk around onto something "hot"
    can't replicate it at idle.

  6. 08EscapeHybrid

    08EscapeHybrid Moderator

    That engine setup is prone to failure of the intake manifold gasket if it still has the original style gasket. Mine just failed 2 years ago, and I had it replaced with a revised version... Fel-Pro makes what they call the "Problem Solver" gasket.

    When my gasket failed I had unexplained oil loss (used 3 quarts in 2,000 miles, where it usually only used one in 5,000), as things progressed, the engine ran rougher and rougher, and the check engine light finally came on right before I had it serviced.

    My mechanic replaced the cap, rotor, plugs, and the intake manifold gasket.
  7. phoebeisis

    phoebeisis Well-Known Member

    I SURE hope that isn't it.
    About 4 years ago my intake manifold gasket DID fail-
    losing coolant-JUST to the outside
    I was feeling lazy and didn't want to buy a harmonic balancer puller-
    but mainly lazy
    A had a guy-advertised on CL- talked a good game-fix it
    he had an electric wrench-use it to speed things up
    I didn't watch too closely-but apparently he used it to install nuts and screws-
    DIDN'T start them by hand-he used it to start and finish them
    What a freakin mess-
    few days after he left the plastic cam cover was leaking like heck-cracked
    Over tome I discovered that he had cross threaded the bolt that held the balancer to the crank(I got a new bolt-ran a tap-gently-thru the crank threads-reinstalled it at just 55 lbs(called for 70 lbs-but didn't have much faith in the threads that were left)

    also several oil pan screws
    Anyway-if it is leaking again- I am screwed since safe bet much of what he touched will be cross threaded and won't be repairable-
    I did have him use the Fel Pro 98000 or 58000 or whatever the number of the "super duper" gaskets was
    I looked at my oil-OK
    Coolant-hard to tell since I have had stop leak in it for over a year
    Sure hope it ISN'T the intake manifold gaskets-no coolant out exhaust-
    just a very heavy gasoline smell
    Thanks-forgot about that-sure hope it isn't that!!

    Yeah I was lazy-
    Toxic combination-
  8. phoebeisis

    phoebeisis Well-Known Member

    Talks to a mechanic buddy
    His best guesses
    1) plug wires
    2)bad injector

    I doubt it is the wires-because aI keep getting PO 300 random misfire-if it was one wire-I would get a specific code
    I really really DON'T WANT it to be an injector
    1) that requires pulling off both intake manifolds-many hours-and I mentioned the "mechanic" who did my intake manifold gaskets-he cross threaded several screws bolts-on the few things-harmonic balancer-oil pan screws and drain bolt-I checked-re-did
    so it might turn out that once removed- I might find they can't be re-attached because he screwed up too many fasteners
    2)Lotta work -many many hours-10-14 hours-if nothing is screwed up-back stressing work-maybe $60 GASKETS $40 per injector(might as well replace both

    So I will change-plugs plug wires rotor cap $130 in parts-and cross my fingers
    My guess-best guess-is injector screwed up-
    it happened suddenly-an injector could suddenly fail
    wires etc-tend to slowly degrade
    But the ignition stuff-maybe 1-2 hours-so…it wins fior now-despite being less likely

    Oh once I look at the plugs-it might become even more clear that it is an injector-
    if a group of 4 plugs is "off" it indicates injector-that will suck..

    Diagnosis and Repair by mass parts replacement-GM parts are cheap-easy to find(bright side)
  9. RedylC94

    RedylC94 Well-Known Member

    You can check plug wires for continuity with an ohmmeter before replacing them.

    Have fun!
  10. phoebeisis

    phoebeisis Well-Known Member

    Good idea.
    If I still had a timing light
    I could check and get as crude idea on how random the random mis-fire is
    Don't think I have had one in 25 years.

    All the parts I an changing are at least 10 years old-so they are due
    Unfortunately the injectors seem most likely-most work-maybe 10x more work-the cap rotor wires plugs- couple of hours(gotta pull splash shields)
    Thanks for the tip-I will check the wires-when I pull them.
  11. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    Sounds like "coil". It could be a fuel pump/pressure issue but that usually causes high speed/high load problems but idle and creep-crawl driving is fine.

    Most bad-coil cars and trucks on the road have a circular logo with the F-blank-blank-blank name, and they are famous for out-of-the-blue (pun intended) P03xx codes, mild to severe misfire, no power, stalling or surging at idle, and often exponentially worse when it's wet. Drive a F*** with a bad coil (or coils) - there are many thousands out there - and you will recognize it right away. I have two sister-in-law vehicles in the family and both had sudden coil failures. Car is fine - blink your eyes - car barely runs. Change the coil, you're back in action.
  12. phoebeisis

    phoebeisis Well-Known Member




    Heh I couldn't resist a Slam at the Blue Brand (notice the poetry of that phrase-in keeping with Maxx's puns)
    Especially since since the Suburban has been such a reliable $2950 vehicle(8 years-never left me on the road-despite current problem-water pump- two 2 3000 miles road trips with a leaky radiator AND leaky intake manifold gaskets
    AND little to no proactive maintenance(oil changes-that is it)
    Yeah it is an OWNER PROOF truck.

    F O R D on the other hand …. well they are said to be very good now.

    Coil-hmmm-on the bright side- cheap fix

    My previous post-one of them-suggested ther were two fuel nozzles-this was my buddies slightly dated stuff-the TBI had two nozzles

    I have learned more asbout GMs alphabet soup EFI than I ever cared to
    TBI- 1987 to about 1992-93 FI that at least KNEW how much fuel to inject by throttle position-and RPMS -GEAR- don't think it had a "mass of air sensor"

    CI Central injection-had air sensor AND just one CENTRAL injector(GM FORD-LIKE CHEAPOUT-6 NOZZLES with poppet valve-they all "fired" at once into intake runners

    SCFI Sequential Central Fuel Injection Meaning 4 fired at once them the other 4-But this extra time NOT firing -caused them to PLUG-still just ONE central fuel injector(cheep cheep) with 8 nozzles-poppet valved-plug up
    I have this on my 1998-they are famous for leaky plugged nozzles-causing PO 300 random misfire-the poppet opened mechanically-pressure from injector

    MFI Multiport Fuel Injection- 8 injectors and they are electrically operated "valves"
    many 96-99 5.7s SCFI are converted to MFI- to cure the common SCFI problems-
    In CA- GM blamed some "fuel requirement" and sent out a SB saying to CONVERT 96-99 with SCFI to MFI-if they have more than one episode of "bad nozzles"-
    they offer a full kit-$310(deep discount on ebay) -it is TRUE BOLT ON-
    not "bolt on if you are a machinist and can make a bunch of parts brackets" bolt on

    Anyway-fair chance that once all my ignition parts replacing-now including a coil-
    FAILS-that I will be bolting on a MPI setup

    Oh-pulled #1 plug-wet with fuel-but not bad looking-just .062" after 10 years 80,000 miles-platinum plug-motorcraft I think

    Ran it with #1 wire off- it was BARELY -if any-worse running-
    Now I really really doubt that I am sooo lucky that I pulled the "bad wire" or "bad cylinder with bad nozzle" but maybe- 1 in 8 chance??
    guess I will pull another plug wire-

    Thanks all

    PS Just pulled #5 plug wire-ran considerably worse
    Pulled #1 wire again-just a little worse
    Hmm maybe #1 has a bad wire or bad injector nozzle?
    The code would have been specific to cylinder #1 if JUST #1 was bad-so….

    Oh-according to what I read-
    it "reads" misfires by being able to measure differences in crankshaft speed??
    How many sensors are there on the crankshaft position sensor?
  13. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    Before going too batty with "sensors", you can opt to follow the old Brooklyn Backyard Diagnostic Trick. Ignition off, unplug a couple of sensors (MAF and CTS and maybe one other) to force the ECM into open loop. Leave crank position sensor intact - car won't start without it. Leave cam position sensor alone - car runs without it but is a beeyatch to get started.

    If the car starts and the CEL pops on right away - it's open loop. You can blast all the codes away later with the SGii after you have corrected the issue and plugged the sensors in again.

    With the computer blind/deaf - Does it run better, worse, or the same?

    If it runs the same or worse, the problem is likely not the computer or sensors - go back to the basics - air, fuel, spark, beer to enjoy while checking the first three. It can run worse in open loop because the computer will fight to keep the engine running smooth and clean even with bad/faulty/failing air/fuel/spark, so the ECM masks the underlying problem.

    If it runs better in open loop, the computer is screwing up "normal" thanks to an errant sensor, corroded connector, etc. Then you can start checking for the reason why the computer is making bad decisions.
  14. RedylC94

    RedylC94 Well-Known Member

    No, no, that's


    At least that's the way I heard it, long ago. There were lots of jokes about that brand dating all the way back to the "T."
  15. phoebeisis

    phoebeisis Well-Known Member

    My suspicion is that the sensors-ecm-
    are just fine
    From various internet searches-readings
    The type of injection system I have-central injector with 8 nozzles opened via poppet valves-
    commonly fails-and pretty much does what mine is doing.
    I am going to pull the rest of the plugs-eyeball them-
    still unsure if " gasoline wet plug" is normal or abnormal-
    back when-with CARBS- a gasoline wet plug would be ABNORMAL
    Sure seems "abnormal" to me-but…
    I'll have to give my buddy a call-pick his brain

    Pretty sure D/C #1 wire-only made the stumble a TINY BIT worse
    but D/C #5- MUCH WORSE- so maybe I lucked out-and first cylinder I checked it the BAD one-or more properly-the WORST ONE.

    I will change out that #1 wire Tuesday-new plug wires on the way-if it is suddenly better-than perhaps it is a plug wire-
    Doubt I am that lucky-
    the biggest PITB- and the most expensive-the FI- is probably it
    but I'll do the cheaper easy ignition "parts" stuff first-PLUGS ROTOR CAP WIRES COIL(IF CHEAP)-I have some new plugs-from years ago-
    delco wires $50
    cap rotor-delco $44
    Coil will check
    Time-maybe 2 hours-$100+coil

    New FI setup-$280
    various IM gaskets-Fel Pro-$55
    coolant-have a gallon or so-maybe that is all i will lose
    The FI IM gaskets-10-20 hours of work+ $350
    and I will certainly find stripped fasteners that might screw everything up

    Fingers crossed

    hey another one-
    Ford my favorite brand to make fun of
    My dad swore off Fords-1958 SW- was a lemon-always overheating on the long family road trips from hell-yeah real adventure-stop at gas station
    some poor attendant would CAREFULLY remove cap-steam would hiss out-not water-just scalding steam
    then he would add QUARTS of water
    usually used 1 qt oil per 400 miles also-not too abnormal in 1959-abnormal-but not unusual
    Back then oil was checked a each fill-up because cars COMMONLY used PLENTY of oil-
  16. phoebeisis

    phoebeisis Well-Known Member

    Quick question
    Is a gasoline soaked plug-threads soaked also-always abnormal??
    Guessing yes
    and guessing it means it is mis-firing-
    Pulled #1 plug-easiest to get to
    and it was wet with gasoline-this is after a short run-
    Also pulled #1 wire-hardly any difference in stumble-a little not much
    Pulled #5 wire-stumble worse

    Doubt I am lucky enough to pull the one bad cylinder -especially since it reads PO 300 random misfire-meaning more than 1 maybe more than 2 different cylinders
    The electrical stuff-delco ebay-is on the way
    I will replace JUST #1 wire initially-run it-see if it improves
    Them I'll replace all the wires plugs cap rotor and coil if not too pricey

    So WET PLUG- abnormal right?? could indicate bad injector nozzles-common problem with these GM 5.7 96-99-which is why they changed from SCFI TO MPFI single central injector with 8 mechanical -pressure-poppet nozzles(they carbon crud up from heat CC crap)- to 8 electrical "valve" injectors
  17. 08EscapeHybrid

    08EscapeHybrid Moderator

    Start with the simple stuff... replace plugs and wires.
  18. phoebeisis

    phoebeisis Well-Known Member


    Exactly- "great minds think alike"
    I'm going easy-ish abd cheapish-first
    wires $50 ACDELCO
    Plugs Bosch platinum plus "free I already have them" 100,000 mile DELCO IRIDIUM+$36
    cap rotor ACDELCO EBAY =$44
    so $100 and 2 hours-plugs pitb- pull off fender splash shields-jack up car-come from side rather that back breaking from above-short 5'5" so..

    I will check on the coil-price-and if not too bad-do it too(despite it not being a F O R D

    If/when the above fails-which I fully expect-but those parts are old and need to be replaced-
    I will pay $280 for the MPFI "spider" with 8 actual injectors-bolt on part
    also $55 for intake manifold gaskets
    maybe $30 in coolant etc cetera stuff-fair chance et-cetera will be $100-just because I always underestimate et cetera cost-
    ceteras are expensive
    The above is a good 15-20 hours of back breaking drudgery-
    hope I don't screw something up-have to redo-with new gaskets again

    Anyway this could end up being a $600 and 25 hour episode
    Not bad considering how reliable and old this vehicle is-repair costs last 7 years about $1200 or so-and maybe $40/yr oil change-Mobil 1 10/30 with $10 filter-mobil one or the fancy synthetic Frame

    Thanks all
  19. phoebeisis

    phoebeisis Well-Known Member

    I switched out #1 plug for a newish one
    no change-so it isn't "just" the #1 plug-
    no surprise- PO300 random misfire

    A question- if it is misfiring maybe 5-10%- mpg is off maybe 10-15%
    Just HOW MUCH of this can my cat con tolerate
    Assuming it is helping burn maybe 10% of the fuel?

    Now my trips are short(and I'm using the prius when I can)
    10.3 miles-25 minutes- .7-.8 gallons

    So just how hard is that on a cat con-
    It would be rare for me to drive 100 miles in a week
    65 miles per week-about average during work year

    So how much will the cat con tolerate-
    I'm wondering because my son "needs" the prius 2 days a week
    but some of those days I might want to work(sub teach-get out of house etc-)
  20. RedylC94

    RedylC94 Well-Known Member

    What does your ohmmeter say about resistance of the #1 plug wire? That's a quick and easy test, or at least used to be. Any condensation, carbon tracks, or cracks visible on inner side of the distributor cap? I hope the problem turns out to be something that easy, but won't make any bets.

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