[ds968]

So on any given drive , whether going at a consistent speed, or accelerating , the car suddenly hesitates and stumbles for a couple of seconds , with the rpm dropping for about a second and then returns to normal ..it feels almost as if it would stall if I didn’t downshift and accelerate harder just to keep the rpm up, but if I take it our of gear and just let it idle , it idles perfectly and the rpm stays exactly where it should be without the slightest hesitation / drop.   

 

What could be the cause of this symptom ?

 

I  thought maybe some impurities in the gas, but I read that is a thing of the past and all major brand stations ( Chevron, Shell, etc ) have had cleaners and additives in the gas which goes in their tanks which eliminates that problems.

So what else ?   I’m thinking if the injectors were dirty this hesitation and stumbling would happen much more frequently, not just once in a while as it does now.  

 Also if the ICV would be a problem, it’d be most evident while idling, wouldn’t it ?   

DME maybe ?  The problems I’ve had with DMEs, were  almost always  starting the car, or stalling when coming to a stop .

The MAF is fairly clean , I cleaned it less than 5 K miles ago .

 

What else ? 

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[unknown4u]

Electrical issue. Stops firing. And pickup again. Check the distributorcap. Is the main contact intact? Mine wasn't. Same symptom

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[ds968]

Thanks, will check out the distributor .   I changed the whole thing once , and got new cables at the same time  but that was well over 15 years ago .. 

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[tejon007]

Don't know what the problem is and don't want to just guess.



However, last year sent the injectors to Mr. Injector, since they were 30 years old and had never been replaced or even cleaned to our knowledge. There was maybe one injector that was down on flow. All cleaned up and full flow was restored with quick turnaround.



Fairly painless process and pretty reasonable cost. Maybe something to consider for an upcoming maintenance? You could clean them when you replace those old crusty ignition wires .

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[ds968]

The stage 3 supercharger injectors have a pretty big and strong flow, and according to “Flash” , less likely to get clogged , but one never knows .   

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[Cloud9...68]

This can of course have many causes, but soon after I bought my car way back in 2004, it started exhibiting the same sort of hesitation you're describing.  I went through about 20 items worth of troubleshooting, and it finally came down to the plug wires, which you mentioned.  The way I diagnosed the problem is that I waited until it got dark, then started my engine, let it fully warm up, then just watched it run.  Nothing happened for the first 15-20 minutes or so, but I finally started seeing some stray sparks around the wires.  I replaced the wires, and the problem was completely cured.  No guarantee whatsoever that this is also the cause of your problem, but if your plug wires are 15 years old, it probably wouldn't hurt to replace them.  Good luck - this can be a very frustrating problem to troubleshoot, as there are literally a hundred different things that could be causing it.


Fun fact - I just noticed you joined this forum exactly one day after I did 

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[ds968]

Thanks, I’ll do that in-the-dark test also , before I open the distributor cap and look at the contacts. 

 

 Wow, just one day away, eh ?   Cool !    B) 

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[Cloud9...68]

Yeah, I think we were both among the original group of members.

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[ds968]

Other than the spark test in the dark, I probably won’t get to the troubleshooting for a few more weeks ( the car “rests” for the next three weeks ).   I’m wondering though if I should  check  if maybe a spark plug might have loosened a bit ? Aluminum engine when hot expands just a little , and if the plugs are seated in perfectly torqued ( can’t remember when I had them replaced ) maybe one or more can get just a tad loose to cause a symptom like that ?   Or there’s no way a plug can cause such an infrequent and sporadic hesitation / stumble ?   Am I wasting my time , and best to just focus on other things ? 

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[tejon007]

Just my two cents worth...for free.



We keep a log of everything we do on our cars. This includes regular maintenance and other repairs, including the dates, mileage, parts replaced, etc.



So, this is when changing the oil/filter, replacing spark plugs, ignition wires,the distributor cap, rotor, filters, etc. Then when you combine the record/log with regularly scheduled maintenance, you know how long ago you replaced a part and when it's due again for replacement. This gives you a headstart of where to begin when troubleshooting problems, like what is more likely the problem.



Manuals give you an idea when maintenance intrrvals are suggested. Me? I can't remember shit, and with almost too many cars, it is very necessary to write it down. But, you do you .



IMO, it's very hard to imagine a spark plug loosening itself between its scheduled replacement times. In fact, it's never happened once in my 50 years of working on cars. Once, a spark plug did pull out of an aircooled Porsche cylinder head of a car I owned. But, that was because the previous owner had likely cross threaded and attempted unsuccessfully to repair the mistake (there were helicoil remnants). Blew the spark plug completely out of the head not long after purchasing the car.



Which brings up the subject of always replacing spark plugs on a cold motor, especially/probably air cooled ones. Can you get away with replacingspark plugs on a hot motor that has aluminum heads? Yes, but I think it's a risk not worth the potential cost.



Best wishes for figuring it out!!

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[ds968]

Thanks,  and yeah , I do have notes on the service records envelope which contains the work receipts, for each car , but have not looked at those yet . I just couldn’t remember off the top of my head when I had what done .   Good to know about the plugs though .  I printed the service manuals relevant pages for service intervals and keep it stapled to the back of the envelopes.

 

 

Electrical issue. Stops firing. And pickup again. Check the distributorcap. Is the main contact intact? Mine wasn't. Same symptom

When you say “ main contact “ do you mean on that rotary plate Inside the distributor ( if my memory serves me there’s something like that in there with contact points ) , or do you mean the main wire contact which goes to the coil ? 

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[unknown4u]

The main contact is the central point in the cap that connect the rotor and the cap with each other. That is a grafite part with a small spring. If that is worn out, or has limited contact during the spark of 100.000 volt, the car hestitate.

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[ds968]

Got it, thanks for the details .   Will check it out.      



BTW, I did the  “ spark in the dark “ test last night,  there was no sign of anything like that .   I let it idle for about 30 minutes , almost double the amount of my commute where the hesitation happens once or twice ,   The wires / cables are those pricey blue silicon cover ones, which (  IIRC ) are touted to be “lifetime” longevity .  

Spark plugs were replaced only 18 K mi ago ( NGK Iridium)  as was the wires set.    The distributor was replaced about 40 K mi ago .   But the car mostly sat for a solid five years not having been driven , except very rarely ( maybe once every couple of years ..) I have replaced all those things.  I just started driving it more frequently only this year . 

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[ds968]

Used the Durametric ( hoping it might show a fault ) but no faults codes come up .   Did the  “blink test”,before that , also all clear ..

Damn , I was hoping for something which might save time in troubleshooting..   So when I get around to continuing that, I’ll just check all the items mentioned by members who chimed  in .. 

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[vliegwielolaf1234]

Did you check the sparkplug holes for oil , might be leaking oil in there and cause problems .

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[ds968]

Did you check the sparkplug holes for oil , might be leaking oil in there and cause problems .

I haven’t taken the plug wires off yet to check the spark plugs.   That’s on the list .   

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[ds968]

Update . Pulled the wires off, all plug holes are bone dry !    I also tested them with the torque wrench and they were solidly in there. 

So ( next ) moving on to the distributor cap removal to check the main contact with the rotor.  Maybe this weekend . 

( btw, air filter is clean ,  and I gave the MAF anothef spray shot , just for the heck of it ) 

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[ds968]

Forgot to mention that I yanked one of the plug casings out so hard, the coupling of the two wires / cables came undone .   So the wire from one ( the distributor out cable ) is  not crimped to the metal connection in the other ( the half going to the plug ) .   I presume it can be reconnected , but I’d have to cut the rubber casing to be able to do that , right ?   Too bad I can’t buy a single cable and just replace it , I must buy the entire set , and that’s a waste of money considering these wires are in great shape ( except one now ..  ) with only 18k miles on them .     Strangely enough just coupling that joint back again, without the wire connected still let the car fire up normally, idles just fine, revving it up the rpm range shows no hesitation at all …no idea how that can be, I guess the sparks are so powerful that it works inside that sealed chamber even without solid contact ..  

 

Removing the distributor cap today .   Guessing that after 40k miles it won’t look all that great, and the symptom is caused by what

 Ritchie ( unknown4u ) said in the first place .   Too bad I didn’t do that first before the other troubleshooting and damaging a cable ..

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[oppositelock]

Also check the pick up coil and its wires.  The wires get cooked by the exhaust and can cause an intermittent spark issue.

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[ds968]

Also check the pick up coil and its wires.  The wires get cooked by the exhaust and can cause an intermittent spark issue.

Adding it to the list . 

 

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