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Low RPM 'stumble'


kev
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I have my stock black 88 running pretty darn good now.  I have most of the little pet peeves resolved with exception to this 'stumble' or miss at low rpms.  You feel it some at idle but mostly when cruising at under 2500rpms with your foot barely on the gas.    I've searched this site and found many posts pertaining to similar symptoms but very rarely did the OP say what the final resolution was.   Importwarrier (where is he btw?) said that he had this issue which was the result of a bad distributer cap but I've check that already.   

What I am positive it is not (because I've replaced or properly checked these as a result of dealing with other previous issues and this stumble remained):

  • MAS
  • ISC/TPS/MPS
  • CTS
  • Knock box
  • primary/secondary injectors
  • transmission
  • FPR
  • no codes

Other parts that are new but, who knows, still may be susceptible (but lower on my list to diagnose)

  • NGK 7031 plugs gapped at 28 mils
  • Wires/cap/rotor
  • Distributer rebuilt with new pick-up coil (although is aftermarket)
  • Vacuum advance
  • Fuel Pump with all filters, tank is very clean, lines flushed/cleaned during the build
  • TB seals
  • timing is good
  • jet valves (yes, it still has them) were properly disassembled, cleaned, new seals and orings, and rocker gap is correct when cold

I have a narrowband afr guage on the car (yes I know, better to have a wideband but I had it and it at least gives me some data).  It dances around a bit while this happens but again it's when I barely have my foot on the pedal.  Afrs are good while on the throttle.  

 

I figured I'd ask in case anyone has had this issue and could provide some insight but I was going to start with vacuum lines and ignition.   The vacuum lines are mostly original.   I'm not fond of the spray carb cleaner approach to check them, for I made a fireball years back doing this on a cq, but I'll either start testing them with my vacuum guage or simply replace them all with thick walled silicone lines.   I've already swapped the vacuum booster lines with no change.   As to ignition, I already checked my connections of the wires to the cap and plugs but I'll do a resistance check on the coil and perhaps swap out the plugs with a known good older set.   

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That was my first approach    I've already swapped injectors.  There was no change.   The ones in it now have just been cleaned and flow tested.  

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When it stumbles, does your narrowband AFR gauge read rich for a split second ?

Edited by Caliber308
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It bumps slightly in the rich band and then bounces to mid lean band...back and forth.  If I let off the gas in the slightest bit, it goes lean/off.  If I press on the gas in the slightest, it is about mid to upper stoich.  It's just right on that line where you hold the pedal still when cruising under 40mph.   Anything faster, it is fine because it is over 2500rpms. 

 

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Everything on my Starion is new to close to new. The exception is the ISC. At warm idle it will read at 13 to 12.5 on the wideband, but every once in a while it will dip to 11.5 only for a split second. I still can not figure it out. ECU? a glitch in the O-2 Sensor? Even though I have a Three Wire. Have you thought about the primary injector wiring or plug? Sometimes it can be difficult to figure out. I always heard this when I was young: Spark, Air and Fuel, if one is not right your car will not run correctly. It is trying to figure out which one it is ,that is the fun part.

Good Luck, Bill

Edited by Caliber308
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I read right over the injectors being swapped oops.

Have you put a timing light on it and kept an eye on it while it idles and maybe while you rev it a little?  If you mark the balancer every 90 degrees of rotation you could check timing on each cylinder if you really wanted to drill into it.  

It sounds like you have been through the distributor but it might be worth another go, it might be worth checking the air gap between the reluctor and the pickup for each cylinder, just to make sure things are true.  

For what is worth mine does a similar thing at idle, sometimes it just has a little rough spot.  I think it happens from the ECU chasing its tail trying to keep a certain air fuel target.  I have a wideband and you can see it working to keep a target but the ECU isn't fast enough to really hold a target so it seems to keep the target in the middle and bounces across it.  Mine normally idles at 13.8-14.0 but sometimes as rich as 13.0.  I would probably have no idea this was going on if it had stock exhaust and no wideband.  

Edited by BlueCuda
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I have had the timing light on all four cylinders and simply monitored the flashing.  Nothing out of the ordinary noticed but I like the idea of marking the balancer every 90 degrees.   For the distributer; I was contemplating tossing in the old original pickup to see if there is any change.  I can definitely check the air gaps.   

I have tried swapping 02 sensors.  I had a 2.5" downpipe on it with a single wire new Bosch O2.  I swapped it back to the original downpipe and O2 and I haven't had any changes with exception to getting rid of the issue of the aftermarket downpipe slightly contacting the bell housing and rattling.    I have thought about injector wiring but I'm not sure where to start with it.   I took my time soldering in the new injector clips and used proper water resistent heat shrink tubing.  I even did it when the engine was out of the car and I had all the room in the world to work in.   

I guess I could try swapping in another ECU.  Not sure if I have any more spare 88/89 ECUs, I've had several 87s over the years and I believe all of my 88/89 ECU boxes now have the 87 guts inside them but I'll still check them.     This all makes me absolutely love the MPI and SDS EMS in my other conquest, haha.  

Thanks for everyone's help!   I'll update this as I learn more and hopefully figure it out.  

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If it comes down to thinking it might be the ECU I have an 87 one as a spare I could ship you to try out if you don't have any close.  I would be afraid to sell it though lol.  

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Thanks for the offer but I do have spare 87 computers.  It's 88/89 computers that I'm not sure I have.  I know I have a few 88/89 cases but they most likely have the guts of an 87 inside them (for I simply swapped the guts on my former 87s).  

I really haven't had the chance to dive into this issue yet.  I'll keep everyone informed.  

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