TECH­NI­CAL Q&A

AN­SWERS TO YOUR DIESEL QUES­TIONS

Diesel World - - Contents -

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.

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