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1994 Favorit LXIe No-start


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I'm having a little headache with my '94 Favorit, with the MonoMotronic fuel injection, in that it refused to start yesterday evening. It had started fine in the morning and behaved as normal on the drive to work, it started and ran fine over my lunch break. 4 hours later, it cranked just fine but refused to fire up at all.  I've puzzled and prodded over it (2 hours of that in the dark, with it raining sideways), so I'm asking here for a little sanity check, in case I've missed something in my frustration.

 

I've tried wiggling/jiggling the wiring harness, unplugging/inspecting/reconnecting plugs, in case I have a broken or corroded wire or connector terminal somewhere that I might've dislodged while I was trying to neaten up my O2 sensor wiring; but that has had no discernible effect.

 

(As for why futzing about with the sensor wiring: I'm running an LC-2 Wideband O2 so I can run an AFR gauge, have the secondary output of that set to emulate the original narrowband O2 sensor, and am feeding that signal to the original O2 sensor wiring. It's been running this way for 5 months or so with no signs of problem, I just hadn't gotten around to connecting it properly rather than twist&tape joints, so I was redoing those joins with adhesive-lined heat-shrink crimps. You know how it goes; there's nothing quite so permanent as a temporary fix that works. :blush:)

 

  • It cranks over just as it always has, with no funny noises, and without struggling. I'll check the battery voltage when I'm next poking at it, perhaps put it on charge for a bit; it did seem a little low, at approx 12.5v, but...
  • I have a strong spark from the coil; the spark is weaker at the plug (I hadn't realised the rotor arm has a suppression resistor inside it, so I had fitted resistive spark-plug caps when I made the new copper-cored leads. Will have to fix that.) but it's run with that combination of rotor arm, distributor cap, leads, and plugs for the better part of two months with little in the way of a hiccup. I even tried tightening the plug gaps up a little, in case it was struggling with that.
  • I'm fairly sure that fuel is getting into the cylinder; the plugs are getting wet with fuel. (And, while I was trying in the dark, I tried with ether; not even the barest hint of a pop or sputter.) I have, I note, wiped them dry and tried again to no avail.

 

So I'm a little stumped. As far as I can see, I have air, fuel, and spark; I should at least be getting pops and bangs. Thankfully(!), at least it's doing this within easy reach of my toolbox... :dull:

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Well, it runs again. Just so you can all have a laugh at what turned out to be my screw-up:

Not only had the rotor arm got a resistor in it, and the plug-caps I'd added after the change to copper-core wire, but the spark plugs have resistors in them too. I'm surprised it ran as well as it did -- hell, that it ran at all -- since I changed the HT leads. After going through all those resistors, there'd be next to no spark energy left at all.

 

The fix was relatively simple, ish: Change the plug-caps to non-resistive ones. (Simple and easy for me, anyway, since I've got spark plug crimp tools, terminals, etc. :dull:)

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23 hours ago, Tamber said:

As far as I can see, I have air, fuel, and spark; I should at least be getting pops and bangs.

Having air, fuel, and spark in general is not always the recipe for success. What I mean is that you need the right amount of them to have good ignition and combustion. For instance too much fuel doesn't ignite. So is the case with a weak spark. Just for the theory sake I will add that an engine needs good timing and good compression too to start.

 

I was reluctant to reply yesterday because I realized your information is not reliable. Today I don't see any better situation. You offered us an explanation that doesn't hold water. Basically you mentioned what big surprise was for you to find out there are resistors inside rotor arm, ignition leads, and spark plugs. You attributed the no start issue to that discovery. Well, I have two problems with your explanation:

  1. There are resistors in every item you mentioned. From factory. It's normal. How about that?
  2. How do you explain this quote? "...it's run with that combination of rotor arm, distributor cap, leads, and plugs for the better part of two months with little in the way of a hiccup."

While I agree that resistorless spark plugs improve the spark a little, I don't think you pinpointed the real cause or you failed mentioning other mistakes you did. I don't know what made you change the HV leads with DIY leads in the 1st place. Anyways, just for the sake of true information, I encourage you to check further on ignition transformer and distributor area. This time use a spark plug tester.

Edited by RicardoM
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Quote

Having air, fuel, and spark in general is not always the recipe for success. What I mean is that you need the right amount of them to have good ignition and combustion. For instance too much fuel doesn't ignite. So is the case with a weak spark. Just for the theory sake I will add that an engine needs good timing and good compression too to start.

True enough. But you might be surprised at just how far outside the ideal you can go, and still have it show some signs of life, as long as it's got fuel and spark -- even if the compression is low and the timing is wrong.

 

Quote

There are resistors in every item you mentioned. From factory. It's normal. How about that?

 

Not in every item. The spark plug caps on the 'factory' leads did not have resistors; they were just plain metal connectors just like the ones I have now fitted. However, the 'factory' leads were graphite core, which supposedly have a rough resistance of about 1kOhm/foot; and I was replacing them with copper-cored leads, so I figured that I should perhaps use resistive plug caps to reintroduce that resistance, in the sake of noise suppression.

 

Quote

How do you explain this quote? "...it's run with that combination of rotor arm, distributor cap, leads, and plugs for the better part of two months with little in the way of a hiccup."

 

It certainly seemed to run fine, albeit rather rich; which I attributed to the cold weather of this time of year, but was probably due to a weak spark not fully igniting the fuel-air mix in the chamber. There was some oxidation/erosion on the terminals inside the cap, which might've been a steady build-up over time; but cleaning that back didn't change the no-start, and I didn't take any pictures of it, so *shrug*

 

1 hour ago, RicardoM said:

While I agree that resistorless spark plugs improve the spark a little, I don't think you pinpointed the real cause or you failed mentioning other mistakes you did. I don't know what made you change the HV leads with DIY leads in the 1st place.

 

It still has resistor spark plugs in it; I merely changed the spark plug terminals to ones without resistors. (I had also changed the rotor arm and distributor cap prior to this, but without any apparent change; so I hadn't mentioned it. Didn't realise I was going to be answering to the Dominican Inquisition.) As for why I changed the leads: The ones that were on, were falling to pieces; the terminals were corroding quite badly and were coming off the ends; and it works out about the same, price-wise, for me to make leads than it does to go buy another ****ty set online. (Yes, yes, I know; only if my time is worth nothing.)

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I am not interested in what mods you consider to be good for your car. All I can say is you haven't done a job too good or at least you didn't describe well what was the real cause of the no start. To me it appears you're contradicting yourself repeatedly.You can't have both a 'strong spark and a 'weak spark' at the same time. You can't have an engine running good for two months yet suddenly refusing completely to start while no part has failed. All that while you keep insisting that I have no idea how broad fuel/air mixtures an engine can ignite or try to ignite... For somebody that failed starting the engine for two hours and asking for a sanity check, you seem having suddenly pretty strong convictions about how an engine should be diagnosed and fixed.

Edited by RicardoM
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