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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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I only had a chance to work on it for about an hour this past weekend.   I checked the resistance of the coil and it was good.  Even tried swapping it with a spare and no luck.  So it's not the coil.   I swapped the plugs with an older set I had, no change.    

Although I did find something of interest; the #4 plug looked brand new.  The other three had minor carbon, as expected, but this last one was clean.  Forgot to snap a picture but I can because the plugs are sitting on my bench (the old set is currently in the car).   I definitely think this is a big clue to my problem.   It's not leaking water, I did a thorough leakdown test before firing it up and was very focused on the status of the coolant system during it.   Although I didn't check specifically for spark (was working by myself), I put the timing light on the wire and it was reading a consistent rpms.  So I'm fairly confident I have spark to this cylinder.    My mind is going to the jet valve again (it could be sticking) or possibly a vacuum leak.   The brake booster taps into the #4 runner...it's a possibility.   And yes, I capped the vacuum nipple for the automatic trans when I did the swap.    

More to follow.  I'm going to look into the vacuum situation first before I jump into the jet valve possibility.  I'll do a compression test as well.     

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  • 2 weeks later...

I messed around with this a bit more.  

I bypassed the brake booster and took it for a drive, no change.  I swapped the resistor for the injectors, no change.  Swapped the ECU with a good spare, no change.  So those are off the plate.

I noticed that with the old plugs, it ran much worse and the issue was so much more amplified.  It's also worse in open loop (engine still cold)  This led me back to ignition.  Put the new plugs back in and it made it much more driveable again.  I then focused on the distributer.  I tossed in an old spare with just swapping out the vacuum advance and an oil seal.   Took it for a drive and it originally was great.  The AFRs still bounced around a bit but stayed in the stoich band.  I didn't feel the cutting out like I did before.  Even idle was better.  I was feeling really good until I decided to wind out a few gears.  After returning back to normal cruising, I feel the miss again.  Although it seems to dissipate or simply get a little 'better' with time.  This was with a 15 minute drive, so I'll be driving it some more to see if this trend holds.  Interesting...   

Next up are the vacuum lines, I still haven't messed with them. 

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I'm getting about two bars rich on the narrowband when I'm WOT...seems normal compared to past cars that I had with this same gauge on it.   I still am not ruling out a leaking injector.  Even though I've tried two sets and had this set cleaned/tested, they are still 35 years old.  

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Seems like a injector problem. I could see not having a problem at WOT because of full fuel demand. Perhaps it is time (with 35 year old fuel injectors) to upgrade to Delphis and maybe a Wideband AFR gauge. I know it costs money, but the best investment I made on my Starion was the Maf-Translator set up. There is nothing like having control over your Air/Fuel settings, and having a way to read them via a wideband gauge.

 

Bill

Edited by Caliber308
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On 8/29/2022 at 11:57 AM, kev said:

I'm getting about two bars rich on the narrowband when I'm WOT...seems normal compared to past cars that I had with this same gauge on it.   I still am not ruling out a leaking injector.  Even though I've tried two sets and had this set cleaned/tested, they are still 35 years old.  

I have suspected mine leak a little.  If my car has been driven already for the day it barely even cranks, it fires on the first compression stroke.  For a car that doesn't pre prime the pump it feels like the injector has dripped just enough fuel in it to fire off.  It start normal on a cold start of if it has sat for several days.  

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9 hours ago, tux said:

Narrowband is kind of worthless for anything.  I had one and it was impossible to tell what was going on.

I wouldn't say that.  It is better than nothing, it tells me the story.  I'm not buying a wideband for this car. 

I'd spend $250 for a diagnostic tool that I could use any any car but to toss that in this particular vehicle just doesn't make sense to me.   I don't plan on doing any type of performance enhancements to this particular car. What will it do for me?  Prove that it is missing?  I already know that.   Point me to injectors that I already suspect?   The narrowband already did that.    I'd rather spend the money on a heated 02 sensor and a set of new injectors.   

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Makes sense then.

 

All i ever managed to glean from my narrowband meter was that the light moved around like i was driving Kitt from nightrider.

If it helped point to an issue great!

 

Mine was more like an indiscernible light show.

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Members are just trying to help.  If you don't want to buy a Wideband, that is fine. Kev, your acting like you have made your mind up on what you are going to do, but your still posting about a problem you have. I use to have problems with the narrowband not giving me the proper information. Once I installed a Wideband and the MAF-T, the problems were solved by Air/Fuel adjustments with proper and accurate Air/Fuel Ratios that I could see in real time with the Wideband. Also, If you install Delphis you need to up the fuel pressure to at least 43 to 45 psi for them to atomize properly. So see, even more money needs to be spent to install a RRFPR and a Fuel Pressure Gauge. I checked on another sites archives and found a post with a problem similar to yours, it turned out to be a vacuum hose.

Just my observation,

Bill

Edited by Caliber308
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  • 2 weeks later...

I'm keeping the thread up to date in my status.   I do have a plan, yes, it is not acting, but I do consider suggestions from others while I execute that plan.   But I'm also trying to illustrate my goals with this car...to keep it as close to stock as possible.   So if I don't take suggestions involving modifications, you know why.   Yes it may take me more time, and time is indeed valuable, but it's generally enjoyable time.    It's just putting off the bodywork I need to do on it but I've already made my decision to wait until the spring to paint this car, it's too late in the year to worry about it now.  

I haven't worked on this issue for a week or so because life has been very hectic but both my son and I have been driving this car regularly.   The problem is there consistently but it is much more subdued since the distributer swap.  Although it does appear to be more exasperated after getting on it a bit.   My son doesn't even notice it and thinks I'm crazy for being so fixated, but that is me, I know it is not running as good as it could be and I'll continue diagnosing it.     

New vacuum lines are on my list, just haven't bought them yet.  I do have another set of injectors on my bench to package up for sending out to be cleaned but I'm holding off until I try a few more things.   I also want to look at the EGR some.   

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