Here's the problem: at 2000 RPM or more, spark delivery fails on cylinders 2,3, and 4. Spark is delivered only to cylinder 1. The problem is random, but when it occurs, it is always at about 2000 RPM.

At first, we thought it was an in-the-tank fuel pump issue or air getting into a return line. Fuel pump is OK. New crankshaft position sensor.

We replaced the fuel filter, checked the high pressure pump and all fuel lines and electrical connections.

Replaced distributor cap, rotor cap, wires, plugs and ignition coil;

Replaced a failing Mass Air Sensor.

No error codes from the Jetronic unit.

Ignition module checks out OK, but something is telling cylinders 2-4 to not fire above 2000 RPM.

It has me and any number of Volvo mechanics perplexed because no one can think of anything that would specifically cause this problem.

I rigged an old-time oscilliscope to the output of spark coil and got a very nice saw-tooth waveform until about 2000 RPM at which the spark is simply not there on cylinders 2,3 and 4, but is present on 1.

Anyone ever see something like this?

 

the only difference between spark 1 and the others is the distributor cap and wires. what kind of wires did you use? I like the OEM style Bougicord wire sets.

unless the distributor rotor isn't turning... you might just take off the cap and crank the car over and verify the rotor is turning. the distributor shaft is driven by a bevel gear on the intermediate shaft (which also drives the oil pump). the intermediate shaft is driven by the timing belt.

 

The distributor is turning. I'm not sure the source of the parts the mechanics used. I will ask them. The odd part is that it is an intermittent problem which occurs only at the noted RPMs.

 

if you have a 12V LED test light, hook it to coil pin 1 (I think thats a red-white wire) and ground, slowly rev the engine, it should be flashing twice per turn of the engine, see if the flashing slows way down when you exceed that ?? RPM threshold where only cyl 1 is firing.

 

Maybe you have a weak coil or improperly gapped plugs.

 

OK, it's now going on two months and the mechanic is still putzing around with the car.

He still can't figure out why its not firing on the 2,3, and 4 cylinders. Now the problem is still random, but now occurs at any given RPM. Mechanic claims it's one of the sensors, but one that will not throw an error code (which is a bunch of BS if you ask me).

What is the chance that the ignition module is bad or there is a bad electrical connection to the ignition sensor?

Every time I take this car to a mechanic, they work on everything except the obvious problem, charge you a heap of money, and only when you say 'just give me my car back and I'll take it to Volvo" do they suddenly fix the problem? This is the fourth mechanic who has kept the car for at least two months. Obviously, the guy most not know how to work on this car?

 

Take it another shop, hopefully an old Volvo one... Too many weird things going on to venture an opinion here.

 

OK, took the car to Volvo. Here's what they found...

A big fat nothing. They said there is nothing they know of to account for this problem, which has now gotten a bit more bizarre.

Everything checks out OK. No error messages, no nothing. Now, spark cuts out in the same cylinders at exactly 2500 RPM. And even Volvo can't figure it out and they said they have never seen this problem before and know of nothing that would cause the spark to cut out at exactly 2500 RPM. There conclusion is that this condition is impossible and that there is no solution. They said they cannot fix the problem and it cannot be fixed because there is nothing that can cause this problem.

Go figure.

 

did you try my LED test light thing out? put the test light between coil pin 1 and ground, and slowly rev the engine, the blinking should speed up evenly with the RPMs, if there's a discontinuity at 2500 RPM where it suddenly blinks slower, then the problem is with the ICU or CPS. at 2500 RPM there should be 2500*2/60 = 83 blinks/second, which is pretty steady. if the ICU is only telling it to fire once every 2 full turns, then that would be about 20 blinks/second, you'd definitely see that slowdown.

 

Did the LED thing.

At exactly 2500 RPM, cylinder one continues to fire and the spark to cylinders 2,3&4 suddenly cut out. When the RPM drops below 2500 RPM (due to the offending cylinders cutting out) the RPM comes up to 2500 RPM and once again cylinders 2,3,4 cut out again. Cylinder 1 continues to fire.

On this 240, according to the service manuals, the only things computerised are the Jetronic electronic fuel control unit and the Electronic Ignition Unit/Module. The Jetronic EFI unit bench checks perfect. There is also a sensor (either a Hall or other type) that sends information to the ignition module and the Jetronic EFI unit, but why at exactly 2500 RPM and only on cylinders 2,3, and 4? This is what has all mechanics perplexed.

When they pulled the spark plugs and re-gapped them, the problem became consistent and always not occurs at 2500 RPM. Before, it was random and at a lower RPM.

Every discrete component and the associated wiring checks out perfect. All the wiring checks out. The only thing we haven't checked is corrosion on any one of a number of fuses.

I'm at the point that I am trying to find ways to retro-fit the whole shabang to a purely electro mechanical system if possible. Either that, or beef up the suspension and drive train and drop an old Chevy 350and tranny into it and have done with it.

 

the timing sensor is the crankshaft position sensor mounted on the top of the bell housing at the back of the engine, reading timing holes drilled in the flywheel or flexplate.

offhand, I'd try and find another EZK116 ICU and swap it. there's two versions for 1989+ 240/740/940, with and without EGR... on the 240's, they are mounted on the passenger side of the firewall behind the glovebox. on the 7/9's, I think they are on the drivers side of the firewall. 240's before 1989 used a completely different ignition system

 

I was thinking the ICU might indeed be the problem, given that it picks a specific RPM and it only causes three of the cylinders to not fire.

Now, the fun thing about this '89 is that it was an early '89 that was schlepped over from Sweden a couple of years ago. The ICU on this critter is located behind the kick panel on the right side of the car (not on the firewall), passenger side just above the Jetronic EFI unit. It's a little black plastic box. You pull off the cover just in front of the passenger front door. Very easy to get to, not an expensive item as they are still in production.

This particular vehicle apparently uses the crankshaft position sensor to also detect engine RPMs (some 89's I have seen have a Hall sensor in the distributor cap). If either sensor fails, it is usually a total and complete failure unless the electrical leads short out intermittently (a lot of '89s had the problem of crankshaft position sensor failure due to the fact that they forgot to put the rubber grommet in the firewall where the CPS leads passed through.

I also thought of a really silly potential cause for this problem that was suggested to me by a SAAB aircraft engineer - check for corrosion on the contacts of any fuse in the fuse bus that might even be remotely connected to the ignition system. It is a Bosch electrical system, after all, and Bosch was famous for using electrical connections of substantially different qualities in terms of the electromotive scale of the metals used (such as brass against aluminium, copper against zinc, etc.,,,).

 

89 was the first year for LH 2.4, which is the configuration that uses the crank position sensor, previous cars were LH 2.2 and used the hall sensor.

the best thing you can do to your 240 fuse panel is, order sufficient Flosser brand brass/copper fuses, and when you replace the fuses, use a soft pink pencil eraser to clean each holder spring right where the point of the fuse goes. These cars used tin fuses which corrode and fail. the copper/brass fuses are much better. 740/940 use modern blade fuses and don't suffer from this problem.