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Discussion Starter #1
Hi I'm new and in need of help I just purchased a 1994 chevy silverado k2500 with a 6.5 td drive by wire with a remote pmd behind the grille it will start right up cold and idle fine first try how ever once it warms up it will jump from it's low 800 rpm idle to a 500 rpm idle and wander between the two but about a minute of that then it drops to 200 rpm and spike to 1200 rpm and does that for a little while before dying altogether I've checked the fuel system for air wired the lift pump to run any time the ignition is on and some one told me to unplug my optical sensor and that fixed everything for about a week now it won't do anything without the optical sensor plugged in the injection pump is new and works I tried the luke warm water trick and it makes no difference at all if anyone can help I would appreciate it very much
 

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Welcome to DieselPlace
Wiring the lift pump to run is not a great solution.
Proper diagnostic testing can rule out a bad lift pump.
When you change out the lift pump you should also change out the Oil Pressure Switch because the switch provides power to the lift pump. If the sensor is bad you have power issue to the lift pump.

Look in the 6.5L FAQ page for links on how to perform proper diagnostics.
 

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Might be a air leak into the fuel system perhaps but what you could do is run the engine from a clean container of fuel bypassing the fuel tank and filter housing altogether to just find out if it now runs like it should.

I’ve done that various times on these types of trucks to pinpoint problems.
 
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93 C3500, Crew, LB, Duals, F code with 141 Block. Replaced 90% under the hood this year.
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If it wanders 1000 rpm on its own, that is a classic indication of internal damage in the roosa-master DB Injection pump. A 1000 rpm swing on its own is usually mechanical with that pump. Just my 2 cents. Go through all the documented diagnostics listed on this site, but I'd keep that in mind. Please keep us apprised as you get findings. Best of luck.
 
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Discussion Starter #5
Thanks for replying so I did a full fuel system after purchasing the truck flushed it did filters and the lift pump the oil pressure sensor was replaced by my local shop before the truck was parked a year ago with the same problem I purchased it a few weeks ago the rewire on the pump was to see if the oil pressure sensor was tripping it or not so I the all fresh fuel including the in line filter before the lift pump the injection pump and pmd relocation was done by oregon injection
 

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Thanks for replying so I did a full fuel system after purchasing the truck flushed it did filters and the lift pump the oil pressure sensor was replaced by my local shop before the truck was parked a year ago with the same problem I purchased it a few weeks ago the rewire on the pump was to see if the oil pressure sensor was tripping it or not so I the all fresh fuel including the in line filter before the lift pump the injection pump and pmd relocation was done by oregon injection
If the tank sender was replaced and an incorrect tank sock was used you will have a fuel restriction.
 

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Discussion Starter #7
I used only ac delco and styndine parts
 

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Please list trouble codes, if any.
 
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Also When I get the truck running the check engine light does not come on unless I'm going over 30 mph the truck is obd 1 my local shops and all auto parts stores can't get there readers to power. Up when plugged in so no codes sorry
 

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Discussion Starter #10
When I get back from work I'll try to run it off a Jerry can and see if there a difference will keep you guys updated
 

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Also When I get the truck running the check engine light does not come on unless I'm going over 30 mph the truck is obd 1 my local shops and all auto parts stores can't get there readers to power. Up when plugged in so no codes sorry
All you need is a paperclip and the info in this link to read any codes: Diagnostic codes OBD I
 

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You need to check trouble codes with method in above post, before doing any more diagnostics. May save a lot of time, trouble, and blindly throwing parts at it. Post up your results.
 

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Discussion Starter #13
Ok I'll try that tomorrow thanks guys
 

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The fact that messing with the optical sensor affected the problem seems to indicate that it is related to the timing. This would also make sense since the problem appears after the truck warms up and the coolant temp switch changes the timing. My guess would be the crankshaft position sensor, but the codes will lead you in the right direction.
 

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Discussion Starter #15
Haven't checked the codes yet but I did get it to start there replacing the sewer system in the town I live in and asked me to move it so I tried and it fired right up but just before the rpms start to jump the oil pressure drops and rises twice on the third drop the rpms jump thanks for the help I haven't had time to check he codes I'll do that this afternoon
 
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