Mysterious Misfire

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SES Light
The Dreaded Service Engine Soon Light

The Mini-Van known as Ford Windstar, produced between the years of 1994 and 2003, and equipped with the 3.8-liter V-6 engine (V.I.N. code 4) is typically a reliable form of family transportation. This particular Windstar, which I personally encountered, provided me with an education which I now pass on to you.

By the time this 2002 Ford Windstar found its way to my shop it had made the rounds. First, it was taken to another independent garage, then to the local Ford dealership. This is not uncommon for us, as we are subject to get vehicles that are problematic and have been to several shops, where repair efforts have proven unsuccessful.

Initially, the owner of the van complained of a “service engine soon” indicator, which had recently illuminated in his instrument panel. He also commented that, under acceleration, his van seemed to stumble, or miss.

At the first shop, the van which had over 100,000 miles on the odometer, received a maintenance tune-up. According to the mileage the van was due for this repair. A new set of double platinum spark plugs, premium spark plug wires, a PCV, a fuel filter, and air filter were installed. Additionally, the throttle body was cleaned and an injector service was performed. The codes were cleared and the malfunction indicator lamp was deactivated. Whether or not the vehicle was test driven is not known to me.  What is known is the fact that the service engine light was re-illuminated on the ride home and the customer complaint was still present.

After spending nearly five-hundred dollars on the tune-up, this customer was in no mood to be provided with an estimate for additional repair work, however, that is precisely what he received from shop number one.

The van owner chose to decline the additional repairs and took the van to a local Ford dealership. Not being familiar with the exact circumstances of this transaction, and not desiring to cast aspersions on the dealership, I will simply say that the van owner was sold a PCM, which had to be installed and reprogrammed. The cost of this repair was slightly greater than the cost of the initial repair.

The owner of the van paid the dealership and drove away. Before he could return to his residence, the service engine soon light was illuminated and the initial customer complaint remained. The van was returned to the dealership where he received an estimate for additional repairs.  He was extremely unhappy, dissatisfied and discouraged.

After receiving a referral from one of the technicians at the dealership, the van owner delivered the van to my shop. Being aware of the previous repair history, I anticipated the vehicle would be diagnosed with a blown head gasket, burnt valve, or worn piston rings.

A quick scan indicated that the engine was misfiring on cylinders number one and four. Further diagnosis indicated that the number one cylinder was misfiring with a greater degree of regularity than was the number four cylinder.

A compression test yielded favorable results. All cylinders had approximately 185-pounds of compression. The ignition coil, spark plugs and wires were firing properly and the fuel injectors were operating normally. The van never misfired at idle, only while driving.

I removed the vacuum supply line from the exhaust gas recirculation (EGR) valve and drove the vehicle. There was no misfire present and no service engine soon light illumination.

After removing the upper intake assembly, I discovered that when the EGR valve was in the open position, exhaust gases were distributed into each cylinder via a single tube with individual cylinder openings. Four of the six cylinder delivery openings were completely clogged with carbon. Only the openings on the two misfiring cylinders were clear. All of the exhaust gases, from the EGR system, were being delivered into two cylinders instead of six cylinders, increasing the air delivery to cylinders one and four; rendering them ineffective.

The tube cleared of the carbon using an ozone friendly solvent and compressed air. The upper intake was reinstalled using new gaskets, the codes were cleared and the service engine soon light was deactivated.

An extensive test drive proved the repair to be effective and the vehicle was delivered to a very happy customer.

Often misfire codes are an indicator of faulty ignition parts, but don’t take anything for granted. Always prove out your diagnostic theory completely, and bear in mind that a malfunctioning EGR system, a faulty fuel injector, or compression problem can mimic ignition component failure and cause the PCM to store a misfire code.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.

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