A Hard-Start Mystery

This 26-year-old Jeep Cherokee requires a bit of investigation, but if you follow the clues you’ll reach a solution.

If you’ve been working in an independent shop for a while, you’ve seen your share of Jeep Cherokees come through your door. Throngs of four-wheel-drive enthusiasts love them, and the vehicles come to us in all manner of modification. My latest encounter was no different.

1990 Jeep Cherokee, 4.0 liter engine

Figure 1: Top five hits for my Jeep Cherokee.

Figure 1: Top five hits for my Jeep Cherokee.

When evaluating a 26-year-old, modified four-wheel-drive vehicle, I keep an open mind. This hard-starting Jeep had a lift kit, aftermarket wheels and tires, added audio, lighting components and a multi-owner history with no record. On all these types of Jeeps, I look for symptoms caused by water intrusion (driving through rivers), wiring ripped out (driving through ice, mud, rocks and branches) and general weariness (high-mileage and active lifestyle).

This Cherokee had the requisite amount of mud-caking on the suspension, and I took all of this into account as I attempted to reproduce the problem. Every time I tried to start the Jeep, it took 8-to-10 seconds. I decided to perform all the engine performance checks for tune-up condition and a thorough visual inspection.

Spark plugs, wires and all the filters were older but not among the worst I’d seen. So I researched probable causes for hard starts. Figure 1 shows the list of top hits for hard starts and no starts for these Jeeps, with no start, no spark and hard start in the top three categories.

Figure 2: Full key-on pressure.

Figure 2: Full key-on pressure.

Under hard start, there’s a strong indication that I should check the fuel pump. Key-on pressure jumped up to 35 psi, but dropped almost immediately to zero, which indicated a leak. Figure 2 shows immediate pressure, and Figure 3 shows it dropping to zero on key release. Following diagnosis, I discovered that pinching the pressure line to the fuel tank maintained pressure but releasing my clamp allowed it to dive.

My Jeep needed a fuel pump with a good check valve. Could this be the root cause of the hard start? In my opinion, the answer was no because pressure would rise to 35 psi, but the engine would not fire for another six seconds.

There’s a lot of discussion about camshaft and crankshaft sensors being faulty with hard starts. But “sync” was also a possible issue. Figure 4 shows the injector light and spark tester installed. When I cranked the engine, I had fuel pressure and spark, but my injector light did not flash for 6-10 seconds. However, as soon as it did, the Jeep fired up.

I then had a diagnostic path to follow, and research indicated that I should have a 300mv alternating current (AC) from the crank sensor and a strong square wave. I had about 240mv from mine and wondered if this could be the cause. With previous no-start vehicles, I had used a procedure to “modify” the crank sensor by removing it, elongating the two bolt holes and reinstalling it to get the magnet closer to the flex plate and increase voltage.

Figure 3: Key-off pressure.

Figure 3: Key-off pressure.

That procedure is a known fix and easy to do. So I performed it on the Cherokee and increased my voltage to around 400mv but with no noticeable increase in start time.

I next checked the voltage and pattern from the camshaft sensor in the distributor, but it had acceptable voltage and a solid pattern. So what next?

The third most likely culprit on these 4.0 Jeep engines is loss of distributor sync. I set the engine to top dead center (TDC) on the compression stroke for cylinder No. 1, found the No. 1 tower on the distributor cap and scribed a line straight down to the base of the distributor. I then indicated the distributor base with a reference mark for the No. 1 tower.

After removing the distributor cap, I checked the position of the distributor rotor. The rotor should just be leaving the No. 1 tower, with the trailing edge of the rotor .020 inches from the No. 1 tower. On my rotor, the gap was much larger.

Figure 4: Injector and spark testers installed.

Figure 4: Injector and spark testers installed.

Since the distributor is nonadjustable from the factory, I made an adjustable one by removing it from the vehicle and grinding off the index slot. Then, I reinstalled the distributor and adjusted it to achieve the .020 inches specification, which made the engine start more quickly.

What did I learn from this Jeep encounter? Miles of hard driving can result in many possible root causes for the same symptom. You need to employ a wide angle of vision to nail down the correct cause or causes, especially when the works are caked in mud.

Comments

comments