Have you ever accepted a repair job while second-guessing yourself the entire time? That's what I felt after agreeing to diagnose my friend's 1997 Dodge Intrepid that only had 11,324 miles on it. The reason for my Intrepidation was due to the fact that the car was still under warranty for the “check engine” lamp concern. But, because the dealer had three whacks at hitting the root cause without fixing the problem, my friend gladly turned over the keys to me with that “please, please, can you fix it?” look on her face.

History

This car is young, clean, well maintained, never abused, and her owner takes it to the dealer for all scheduled service. The dealer had pulled a code PO306 (Cylinder No. 6 misfire) from memory, had the car overnight, and sold my friend a “tuneup” consisting of plugs and a fuel filter, but the check engine lamp still illuminated each morning with a one- to two-minute rough running condition from a cold start. On subsequent visits they turned off the check engine lamp, offered a bottle of in-tank fuel system cleaner and cleared the codes.

Baseline

I first drove the Intrepid on a brisk January morning, and I did it sans any diagnostic equipment; I simply performed a seat-of-the-pants test by starting the vehicle and pulling away from the house. The rhythmic pulse of a misfire was evident right away, and the check engine lamp illuminated one minute after startup (Figure 1).

Developing a Diagnostic Strategy

I decided to gather data on this Dodge three ways:

• Pull codes and record a freeze frame during the misfire with my scan tool.
• Search for any applicable technical service bulletins (TSBs).
• Search the International Automotive Technicians Network (iATN) for pattern-case failures.

With a warm engine, I test drove with my scanner installed, but I couldn't re-create a misfire no matter how hard I loaded the engine. But, after letting the car sit overnight, I recorded the freeze frame data in Figure 2: Cylinder No. 6 had eight misfires; I love this feature of OBD II! I also captured some interesting long-term adaptive fuel strategy in Figure 3: The left side was adding 18 percent, and the right side was adding 13.7 percent. This information, coupled with the fact that I couldn't re-create the misfire with a warm engine, led me to believe that I didn't have an ignition misfire, but a “lean” condition involving the left bank, where cylinder No. 6 was located.

I found a TSB for “erroneous MIL illumination with DTCs for multiple cylinder misfires.” This involved erasing and reprogramming the powertrain control module and slapping a “modification” label next to the VECI label. I was not convinced that this was my condition because it is consistently tied to cylinder No. 6, but I made a mental bookmark, just in case.

A search of iATN revealed a very interesting pattern. Many techs posted a “help” message that sounded eerily similar to my customer concern: “Found code PO30X, checked for vacuum leaks, compression test was OK, MIL comes on after one to two minutes of driving from a cold start.” In fact, many of the “fix” posts involved a leaking intake manifold gasket, but what concerned me was that all of those 3.5L engines had 60-90K miles on the odometer, not the 11K that mine had. I made another mental bookmark and continued with my testing.

Intrusive Testing

I traded my Snap-on scanner for my diagnostic and reprogramming tool (DART), the DaimlerChrysler factory tool; well, for the Chrysler side anyway. Turns out the DART will clear codes on this engine, but the Snap-on won't. There's one reason for owning more than one scan tool! I found the same PO306 (Figure 4), and I was able to cause a PO206 (Cylinder No. 6 injector fault) by unplugging the No. 6 injector harness. Just confirming that the self-diagnostic software was fully functioning. I switched back to the Snap-on scanner for the injector balance test (Figure 5), and disabled each injector, one at a time, while observing the rpm (Figure 6). All injectors were dropped from 940 rpm to about 860 rpm when killing them on a “cool” engine.

Even though the dealer had replaced the spark plugs, I swapped the No. 6 plug with No. 3, and installed a test plug wire for No. 6. On a cold start, the PO306 returned, confirming the wire and plug were not the cause of the misfire. We're making progress by confirming what's “good.” Carefully look at all six spark plugs in Figure 7. There is a definite difference in the No. 6 plug (upper right) when compared to the rest. There is a black band of carbon ringing the ceramic, further affirming a fault in cylinder No. 6.

It was now time to try and prove the misfire was due to a lean condition, so I snaked my propane hose into the intake snorkel and watched the fuel trim data while firing the engine from a cold start. Immediately after startup I opened the propane valve, forcing the LT fuel Adaptive numbers from the teens down into the single digits, and I felt a corresponding smoother idle. OK, we're lean, now what?

Confirming the “Basics”

Faced with the low-mileage 3.5L in Figure 8, I just couldn't bring myself to contemplate violating those pristine fasteners, so I performed as many engine performance tests as I could think of. After all, this was a $30,000 car that was still under warranty; I had to go above-and-beyond with my tests to make absolutely sure I had the root cause nailed.

Ohms tested the injectors, and all were in spec at 14.1 to 14.6 ohms. I also performed a flow test of all six injectors using a fuel pressure gauge and injector tester set to the “cruise” mode. In this mode, each injector is opened for exactly 500 milliseconds. Here are the results with beginning pressure/finished pressure for each cylinder:

Cyl No. 5 48 psi/35 psi Cyl No. 6 48 psi/35 psi Cyl No. 3 48 psi/35 psi Cyl No. 4 48 psi/34 psi Cyl No. 1 48 psi/36 psi Cyl No. 2 48 psi/34 psi

I confirmed that each injector was capable of flowing the same amount.

I had to run the full battery of compression tests before proceeding, so here are the results:

Cylinder Cranking Compressiom Running Compression Snap Throttle 1 75 (first puff)/150 55 125 2 75/150 50 125 3 75/150 55 125 4 75/150 55 125 5 75/150 50 125 6 70/140 55 125

Well, I confirmed that I had a mechanically healthy low-mileage engine, so I steeled myself, and decided to pull the upper intake, injector rails, and lower intake. I wasn't confident doing this because I couldn't confirm a gasket leak by flowing propane around the intake manifold, and by my testing, I didn't have a plugged injector.

Mega-Intrusive Testing

Figure 9 shows the upper intake plenum removed. With the injector rails exposed, I loosened them and carefully lifted them up so I could scrutinize the injector-rail-to-lower intake gaskets. I found these bolts oddly “loose” while removing them. That's when I spotted the gasket in Figure 10; when compared to the “straight” gasket in Figure 11, the one in No. 10 (Cylinder No. 6) was visibly migrating out of place. I thought, “Why wouldn't propane seep past this gasket?” Figure 12 (Cylinder No. 6) shows the mounting pad where the injector gasket seats. Compared to the surface in Figure 13 (Cylinder No. 4), there is evidence of a partial seal, or partial leak.

At this point I looked for other evidence that only existed in Cylinder No. 6. The injector nozzle was discolored when compared to the rest, and the intake runner had darker deposits than the rest. There was a definite injector spray pattern or vacuum leak condition here. I performed the injector flow test again, this time with the fuel rail removed, to compare spray patterns. They all produced a fine mist, but injector No. 6 seemed to spray a bit crooked. I decided that this one would get replaced, but before I did, I just had to pull the lower intake to search for signs of that pesky possible intake leak. It was only 10 bolts and a $10 gasket away.

Figure 14 shows the gasket for Cylinder No. 6 in the early stages of migrating when compared to the other cylinders, but it still looked as though it had a tight seal. I was struck by the fact that these gaskets were already moving, and I developed a hypothesis that they were indeed leaking with a cold engine, but snugged up as soon as the engine temperature rose. Perhaps the intake gaskets were victims of time, not mileage.

The Repair

Whether I felt the intake gaskets were the root cause or not, they were going to get a big upgrade. I purchased OEM gaskets (in case warranty was still an issue) and one fuel injector from my local, friendly DaimlerChrysler franchise, coated both sides of every gasket with High-Tack adhesive sealant, and reassembled the engine. I dare you to leak! My final act was Figure 15: Clear all codes and test drive.

As independent technicians and shop owners, we are often leery to dive into a car that is still under warranty, but in a situation where our customer looks to us as the final option, we must trust our testing and training and take care of our customers.

Today, four months later, my friend hasn't had a “check engine” lamp illuminate or a rough idle. "Nope," I said.

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.

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