Volvo AQ140a - weak spark on one cylinder

Hello mechanics! I have a riddle I'm trying to solve..

I'm the proud owner of a 1981 Bayliner 3270 with two 40-year old Volvo 4-cyl gas engines - AQ140a's. I recently snapped a valve spring on one, and had the head pulled and fixed and re-installed. Immediately after the install I noticed unburned gas in the exhaust water. Wasn't there before..

I've checked the internal engine alignment (crank and cam lined up properly). I've checked the timing - looks normal - approx 30 degTDC. But I noticed the timing was difficult to check because the number one cylinder spark is very weak. The timing gun was very intermittent on cyl one. The other three cylinders are just fine.

Pulling the plugs revealed three normal-looking plugs, but cyl one had some gas fouling.

I've checked the compression -cylinder one looks just fine - just under 150 psi.

Because I have two engines, I've been able to swap out the coil, distributor cap and rotor. All plugs and spark plug/coil wires have now been replaced as well. Nothing has made any difference.

Both engines were updated with electronic ignition by a previous owner. So there are no points or condenser.

The only thing I have not swapped is the distributor itself, including the updated electronic ignition module. But I can't see how that would fail on one cylinder only, the rest are just fine.

The engine starts and runs, but I can hear it is not happy, and there is definite unburned gas in the exhaust..

So... what's going on here? Any thoughts would be greatly appreciated!

Just because you have new wires does not mean you have good wires. Swap the number one wire between engines and see if the problem follows. We ran into a similar issue with a pair of crusaders. We were convinced the wires were good because they were new. Finally, out of desperation we put the old wires back on and problem solved.

Thanks Tilt. That's the first thing I tried before replacing the full set of wires on both engines. No change.

Confirm it's not a fuel issue (like the choke sticking closed). It's possible that it's running really rich and cyl 1 is rich enough that it can't fire reliably (once the plug gets gas soaked the spark will be poor).

Thanks HourGlass. But I'm checking spark strength with a timing gun which clamps on mid-wire. Should be unaffected by fuel issues if there are any. And the three other cylinders fire just fine. We've also pulled each plug individually and checked the spark visually outside the engine. Even with a clean new plug, the spark strength is very weak and intermittent, but ONLY on that cylinder.

Once you’re satisfied the ignition side is ok, if valves aren’t seating properly you will not get good detonation. With valve work being recently done it may be suspicious.

Thanks Sunchaser. But I checked the compression on that cylinder and it shows just under 150 psi. Seems kinda normal for an old engine. I would have thought any valve issues would show themselves on a compression test..? I'm pretty sure it's the ignition side that's NOT okay.. I seem to have eliminated everything but electrical.

Congratulations on a methodical approach to the diagnosis!

A process of elimination would seem to point to the distributor. The only part of the distributor which is specific to individual cylinders is (IMHO) the distributor cap. How about swapping caps?

Good luck!
Nick

Swap the distributor electronic innards between engines. There could be something different. I did the electronic change on a 140 (the last Volvo I'll ever own). The rotor could be damaged or the gap narrower, etc.
Also check the engine ground. The starter draws a lot of amps and can power thru a bad connection, but not so electronic devices. Also if it's the original Volvo coil, that can be a problem.

Thanks Nick. It's on my list of things already swapped.

Thanks Lepke. All good suggestions. Another reply on another forum has pointed to the electronic ignition update.. he mentioned the possibility of one weak magnet (out of four) in the spinning 'electronic ignition' wheel which triggers the sparks. One weak magnet would mean one bad spark - all others good. I'll swap them tomorrow.

I'm not certain there are magnets on the "ignition wheel". It really only needs to be ferrous metal with high spots and low spots to create the waveform needed. I think magnets are not used so that it won't collect bits of metal over time. That said, I could be completely wrong. Regardless, one other thing to check is end play on the distributor shaft. The bushings or shaft might be worn enough (or the shaft may be bent) to make the "ignition wheel" drift far enough away from the hall-effect coil to reduce the amplitude of the waveform enough to make the spark weak on one cylinder.


Consider purchasing a hand-held oscilloscope with built in DVM and function generator. It's a super handy tool for troubleshooting these kinds of issues. I have a Hantek 2D72 that I love. They are not very expensive and there are all kinds or videos online to teach you how to use an oscilloscope.



I recently used my Hantek to test an inaccurate tachometer problem on a Cummins boat engine that was caused by a weak tach sender. The scope proved the tach signal coming from the engine was the right frequency, but with low amplitude compared to the other engine. The function generator allowed me to prove that the two tachs had the same response to various frequencies and amplitudes of input signals. For fun, I was also able to estimate, within one tooth, the number of teeth on a Cummins flywheel (I said 160, they have 159 teeth). A new tach sender solved the problem immediately.

Regarding electronic ignition, my understanding is that it uses a capacitor discharge technique to generate the spark and that the function of the lobes on the rotating part of the distributor is entirely one of timing and has no effect on the strength of the spark.

Can others confirm this?

Nick F said:

Regarding electronic ignition, my understanding is that it uses a capacitor discharge technique to generate the spark and that the function of the lobes on the rotating part of the distributor is entirely one of timing and has no effect on the strength of the spark.

Can others confirm this?

Click to expand...

I was under the impression CDI was something that the engine had to be designed for. I think the electronic ignition conversion is simply a transistor switch that applies ground to the negative terminal on the coil then disconnects ground (which causes the magnetic field in the coil to collapse on the primary winding which induces a current on the secondary winding that can only escape back to ground through the spark plug), based on the signal generated by the pickup coil.

Mismanaged - Good point regarding CDI or not,

However, I still believe that the "transistor" which switches the coil current will be set up to have a "snap action", such that the lobe on the rotor controls only the timing of the switching of the current and not the abruptness of it (the abruptness - or rate of change - of the coil current is directly related to the spark voltage generated by the coil).

Nick F said:

Mismanaged - Good point regarding CDI or not,

However, I still believe that the "transistor" which switches the coil current will be set up to have a "snap action", such that the lobe on the rotor controls only the timing of the switching of the current and not the abruptness of it (the abruptness - or rate of change - of the coil current is directly related to the spark voltage generated by the coil).

Click to expand...

You make a great point. I dug around and found this from Pertronix:


"Most of our Ignitor kits are not adjustable and are factory set to the appropriate air gap. Some kits do require setting the gap between the module and the magnet sleeve. In these cases we supple a clear plastic feeler gauge that measures 0.030". This is inserted between the module face and the magnet ring before tightening down the module. As a general rule, the gap can be set to a minimum of 0.010" and a maximum of 0.060". Any module gap between these two settings will provide the same results. The use of traditional steel feeling gauges will not create a problem with the module."


That said, it's still possible that the air gap for the bad cylinder is on the edge of proper function, but it would seem that weak spark would not be a symptom, IMO. Hmmm... What a strange problem. I hope we learn the root cause.

Timing 30 deg TDC? What does this mean?

Timing at idle should be 8-12 deg BTDC. 30 deg would cause destructive detonation.