1994 Volvo 940 2.3 L 4Cyl. No Start 148,447 Miles

1994 Volvo 940 2.3 L 4Cyl. No Start 148,447 Miles

by Peter Landry – Landry’s Brookfield BP

I was called to a shop to see if I could lend a hand with a no start. At this point I’m told the car has spark, fuel, com- pression and wet spark plugs. Sounds like timing, right? They told me they pulled the timing cover and the marks are spot on.

My first test was confusing. I did a cranking current test with a sync probe hooked to number 1 spark plug wire. The results suggest the engine is in time or at least spark is being fired when A cyl. is at TDC. Figures 1 and 2

 

Second test, I hooked up my 5 gas analyzer and the readings shocked me. On one hand I have wet spark plugs, and on the other I have very little, less than 500ppm, HC out the tail pipe. What can possibly cause this issue???

I figure this engine is not breathing. At this time I recall being taught that an engine is just an air pump. The best test I’m aware of to test the engines ability to breathe or pump air is a cranking vacuum test using a vacuum transducer. Since ATS transducers along with the Escope does the conversion of voltage to inches of mercury for me there is no need to use a gauge here. Notice the pulses are even and the pattern is what I would expect to see. However note the scale. The pulses are going above and below zero inches of mercury. Figure 3

Now we’re getting somewhere. My next test compares cranking compression and the firing of number 1 spark plug. As you can see there is a problem here. There are a few points in this waveform that stick out like a sore thumb.

Let’s take a closer look at the pressure waveform. Figure 5

  1. Spark should occur at or before TDC and it most certainly doesn’t.
  2. The towers aren’t even they are leaning to the
  3. There is no exhaust ramp 3

Final diagnosis:

The cam sprocket locating pin was sheared, allowing the camshaft to turn roughly 90 degrees out of time.

On this car, the distributor is driven by a separate sprocket by the timing belt, which is why the ignition and crank timing was still a match in figure 1 and 2. Figure 6