Return To The Motor Age Articles Index Page....  
 January 1999 By Glenn & Mark Giammalvo
This Jimmy was having a rough day. But we couldn't forget that the customer was having one too! 



 Here is an overhead view of what we found once we removed the upper intake manifold. Notice how the fuel from the leaking fuel pressure regulator has washed off the oil deposits that are normally present due to PCV pullover. 



The inside of the upper intake plenum reveals how the fuel leak has also cleaned one side. 


Here is a side view of the fuel pressure regulator. The diaphragm has most likely ruptured causing raw fuel to leak from its vent hole. 



 This is a view of one of the six poppet nozzles when it is pulled out of the intake. Although the nozzles didn't fail on this car, they can fail in the open position causing the same symptoms of "rich running". 


 Although we did not have to test it, the Jimmy's MAP sensor is easily accessible for testing. It would be nice if all the other electronics were within reach like this. 


 Here we have the fuel injector assembly out on the bench. Unfortunately, the fuel pressure regulator is not available separately. The new injector assembly comes as you see it with pressure regulator, hoses and poppet valves. Advise your customer to sit down first before you give them the price. 








  VEHICLE: 1994 GMC Jimmy 
  POWERTRAIN: 4.3L V6, 4-spd. A/T 
  MILEAGE: 55,093 
  SYMPTOM: Hesitation, rough idle, exhaust odor. 

  Every day in this business we face different challenges. Sometimes the challenge is diagnosing a vehicle and sometimes it is dealing with customers. Often these challenges include a combination of both.  This was one of those days.  The customer reported the little Jimmy was running very poorly, especially at idle, and that it had a sickening exhaust odor at idle. The symptoms were similar to a problem he had on this vehicle recently which turned out to be a failed EGR valve and was covered by the factory warranty. To muddy the waters even more, is the fact that at the time of the first failure, the truck was still covered by our own warranty since we had recently sold the Jimmy. 
  System specifics 
  The sport utility in question shares many features with the rest of the GM line and is well known among technicians, not only because of their popularity but because they have had a similar readable data stream for diagnostics for nearly 20 years. The 4.3 liter V6 is controlled by a single Powertrain Control Module (PCM) that handles ignition, fuel delivery, transmission control as well as diagnostic functions. A well known feature of this PCM is the clear flood mode, were injector on time will be minimized during cranking if the accelerator is held down 80% or more. Much less known is this particular PCMs ability to enter a highway fuel mode, where the fuel mixture can be leaned out beyond the control of the oxygen sensor to improve fuel economy. 
  A distributor with an internal pick-up coil and ignition module is used with an externally mounted high energy coil for ignition function. The module controls ignition timing during cranking but is switched after startup so the PCM can control ignition timing with the assistance of two knock sensors when running. A Manifold Absolute Pressure (MAP) sensor is used to report engine load and a separate Intake Air Temperature (IAT) sensor helps verify intake air density. Shared with other GM fuel systems is an oil pressure switch that provides an alternate source of power for the fuel pump should the fuel pump relay fail. 
  The Exhaust Gas Recirculation (EGR) valve is different from a common pulse width modulated vacuum controlled valve. A Linear EGR valve is used which consists of a small 12 volt motor that opens a normally closed pintle through a PCM supplied ground control. A three-wire potentiometer using a traditional five volt feed, is built into the valve so actual position can be determined by the PCM. To aid in diagnosis EGR valve position can be changed and observed through the use of a bi-directional scan tool. 
 The unique air fuel distribution system, called Central Port Injection (CPI) or Central Multiport Fuel Injection (CMFI), is easy to work on but not easily understood at first glance. What appears to be a central mounted fuel injector on top of the intake manifold plenum is actually an intake manifold tuning valve. This butterfly valve incorporates a small rotary solenoid, powered by the PCM through a relay, to separate or join two chambers inside the manifold assembly. Performance is improved by keeping the manifold split into separate runners at low and high RPMs, and open to one large signal runner at the mid RPM ranges. 
  The CPI is actually located inside the intake manifold plenum on top of the lower intake manifold. Fuel supply and return lines sneak through sealed holes at the rear of the manifold with a fuel pressure test port mounted externally. Fuel is feed directly into six separate fuel hoses with normally closed pressure relief or poppet valves at each end mounted in ports facing the intake runners of the cylinder head. Fuel pressure, approximately 50 PSI, is delivered by the central fuel injector to the poppet valves, forcing the valves open during fuel delivery and then stopping fuel delivery completely when the injector turns off and fuel pressure drops. A note of caution here: We heard from a local GM dealership service manager who advised us not to accelerate a Vortec engine that has a running problem. A sudden backfire could cause the upper plastic plenum to shatter causing plastic shrapnel to hit anyone standing nearby. He even reported a case of the plenum pieces penetrating through a closed hood! So be careful. 
  Driving the Jimmy into the shop was all the road testing needed to see the symptoms the customer was referring to. The truck had a decided roughness and surge especially at idle, and the exhaust smell made someone yell "put a pipe on it" referring of corse to an exhaust hose. Self doubt is stronger than common sense so the first thing we checked with the scan tool connected was the EGR valve. The EGR valve position commanded by the PCM was 0% and actual EGR position was also 0%. Running the valve open and closed with the scan tool showed an equal movement in EGR position. Whew... the problem has nothing to do with the EGR valve. Previously when the valve had become stuck open, the drop in manifold vacuum made the MAP sensor believe there was a load on the engine and it enriched the fuel mixture. It is true what they say about computers, garbage in equals garbage out. The scan tool showed "no codes present," however we did observe a fixed high O2 sensor reading, (800mv), while the car was running rough at idle. At first we created a huge vacuum leak by pulling the vacuum hose off the power brake booster to see if the O2 sensor would see the additional air and swing to a lean reading. No dice. So, what did that prove? Let's see...the O2 sensor is stuck reporting rich. We know that because it should have seen the big vacuum leak right? And the rough running is being caused by a lean command to the injector since the PCM is believing the failed O2 sensor's rich report. Well, it was a good plan, rich is this car in the first place? Could the vehicle be so gross rich that the O2 sensor won't see the vacuum leak we created? Sure...any things possible in the automotive diagnostics and repair department. Having been burned by assumptions in the past we tested the tail pipe emissions and verified CO was out of sight (indicating a rich mixture) before going any further. 
  We looked next at the MAP sensor reading but we were not sure if it could be trusted. The MAP sensor read just over two volts at idle, indicating a loss of intake manifold vacuum, but was this the cause or just the result? Remember voltage should be low, just over one volt at idle or high vacuum conditions, and voltage should be high near five volts with no manifold vacuum present. Now, was it a bad MAP sensor showing a load that was not there and thus causing the rich fuel mixture, or was the engine running so poorly that vacuum was low and the sensor just indicates reality?  Well we thought the small amount of load the MAP sensor was showing was not enough to cause the big oversupply of fuel we were seeing so we disconnected the MAP sensor and verified the fuel mixture was still rich out of control. Everything else in the datastream appeared normal so we checked for service bulletins but only found one which listed a specific tester being available to test this style fuel injector. 

  We checked fuel pressure next, probably just because it was easy to do, we did not really expect fuel pressure to be the cause of a rich fuel mixture but did notice it seemed to loose pressure faster than normal. Now we started to expect an internal fuel leak, perhaps from a stuck open poppet valve, so we removed the upper intake manifold for a look inside. Now we could see a lot of things, a small puddle of raw gas sat in the right side lower manifold, further inspection showed raw fuel dripping from the fuel pressure regulator. Pictures do not do this fuel system justice, removing the upper intake manifold is an education in itself. My brother and I joked about the fuel pressure regulator being available separately, and a quick call to the dealer verified our suspicions. Don't you just hate it when you ask your parts man a question and he answers you before you can finish the question? As we described the make, model and engine to the parts counter person at the dealer, his exact statement was: "whatever you want, just don't ask me for a fuel pressure regulator." It was part of the injector assembly and they had a few in stock. The price for that neat little assembly of injector and plastics? Better have your customer sit down. How does $500 sound? 

  Draw a conclusion. 
  One challenge down, now just delivering the good news to the customer. The EGR valve that would have been covered by the warranty still was ok, but the big dollar fuel injector/regulator assembly was not. Dealing successfully with customer conflicts requires two separate processes be completed. First you must honestly evaluate your position, your actions and even any mistakes you may have made and this all must be clearly explained to the customer. The customer does not need to comprehend what an EGR valve does for example, but they should be aware of its basic function and how it relates to their problem. They also should be made aware of any possible warranty coverage that may apply to their vehicle. Sometimes a customer will choose to pay to have a repair done at a shop that treats them well rather than receive a free repair at a shop they have had difficulty with in the past. Secondly after stating your position and hearing your customers, you must separate yourself from the conflict and look at both sides from an independent point of view. Sometimes this is best done away from the customer to create an environment that lets you separate from the conflict. Arguing with the customer for 20 minutes and then deciding in his favor does not leave the customer with a good feeling toward your shop. Draw a conclusion that you think is fair to both sides and present this to your customer. Explain to them that you understand their point of view and you think this is a fair compromise. Sometimes you can even make them feel good about the situation, tell them you value their opinion and you will agree to a compromise they recommend. People are much happier to get what they ask for even if it is less than what you were willing to offer them. In our case we agreed to split the bill in half, because from our customer's point of view this was a repeat symptom and a repeat problem from right after we had sold them the vehicle. Remember disagreement does not come from one person being right and another wrong.  It results from two different points of view of one subject. 

Mark Giammalvo MAT, SAE, L1 
Glenn Giammalvo MAT, L1 

Glenn  &  Mark 
 Return To The Motor Age Articles Index Page....