ANSWERS TO YOUR DIESEL QUESTIONS
I have a 1998 GMC that is equipped with the 6.5L turbo diesel. The engine has accumulated about 198,000 miles. I recently replaced the Stanadyne DS4 electronic fuel injection pump here in our own shop because of what appeared to be an obvious failure. The new replacement pump was supposedly rebuilt by a Stanadyne
shop. The problem is I cannot get my Tech II scan tool clone to set the Tdc-offset (Top Dead Center-offset) which should be in the range of -1.5 degrees. It just won’t do it. The Tech II clone has a GM cartridge that includes the 6.5L diesel in the setup menu, but I still can’t set the timing. Can you help? Ben Chase Greenville, SC You didn’t mention what failed in the original fuel injection pump, but we assume you didn’t have trouble with injection timing. If the problem with the old pump is the same as the new pump, you may have either a very loose timing chain or an intermittent crank sensor problem, not a pump problem. Ordinarily, these timing chains will run to 250,000+ miles without excessive slack if the engine has had its oil changed every 3,000 miles or so. On the other hand, an engine that receives new oil only every 5-6K miles will experience more wear because of the increased soot load in the oil. These 6.5L diesel engines are not as clean as the current high-pressure common rail engines produced since 2001. Soot is an abrasive. The 1996 and newer electronic fuel injection systems will (or can) automatically re-learn timing. Disconnect the negative battery terminals for 30 minutes, then reconnect the batteries to see what develops. Normally the system will require some time running before you’ll know if that helped with timing. We are not necessarily familiar with the various Tech II Scan Tool clones out there, but we do know that the dealer Tech IIS all incorporate a module having the GM diesel package, which is necessary to properly interact with the 6.5L diesel fuel injection system. There may be a difference between the clone Tech II and the dealer Tech II. So assuming the previous battery disconnect sequence didn’t help, we recommend calling a few local GM dealerships to make an appointment. However, we’d first ask to talk to the service manager. Ask him whether his dealership has a good diesel tech who has experience with the 6.5L. We’ve talked to enough service managers through the years to know that they will be upfront about their shop’s diesel expertise. It’ll be worth the half to ¾ hour of shop time to have a GM tech with a genuine Tech II evaluate your fuel injection pump and set the timing—if there’s no other problem. Normally, the TDC offset number you see on the Tech II screen will bounce around, sometimes a lot and sometimes less so. The slack in the timing chain is part of what produces the variability. A worn-out timing chain can produce nearly 4 degrees of variation and a new chain about 2 degrees. The trick to getting a -1.5 degree set using the Tech II is being good at hitting the “set” button at just the right time. A diligent diesel tech will usually require a few attempts before getting a -1.5. Good luck.