Diesel Place banner

6.5l Diesel Rough/Surge/Blacksmoke at idle

23 views 1 reply 2 participants last post by  bk95td  
#1 · (Edited)
Truck: 1994 GMC 6.5L naturally aspirated, DS4 pump, manual trans, ~140k miles.

Background / Timeline •
Truck ran great for about 2 years before issues started. • From 2018–2021 replaced: lift pump, fuel filter, oil cooler lines. • March 2021: CPS was stuck in timing cover → I pulled cover, replaced CPS + timing chain/gears. Truck ran, but often with SES light (limp mode). Sometimes SES cleared itself while driving, then came back at idle. • 2022–2023: On-and-off issues. Sometimes ran for a month, then wouldn’t start. • Nov 2023 – Mar 2024: Truck ran great all winter. Issue came back in spring. • When I did the timing gears I didn’t check/set timing properly — just installed and went. Truck ran fine afterwards, but still had the same intermittent problems.

Recent Repairs / Steps
• OPS replaced. • Water pump replaced | Glow Plugs Replaced • CPS replaced. • PMD: Old relocated unit in bumper (may still work). New PMD currently mounted firewall driver’s side (in use). • Grounds cleaned (intake, frame, batteries). Resistance improved from ~188Ω → ~0.1Ω. • New leak at FFM, but problems existed before leak.

Electrical Findings
• Before 5V was stable, the only consistent code I had was Code 19. • After I fixed the harness and was probing, more codes came up — but I believe that was because my back probes were shorting against the alternator bracket ground and creating false faults. • After finally stabilizing the CPS harness with no probes touching ground, the truck started with no codes. • Right now the truck still has black smoke and a rough idle, with bursts of smoke that seem to time with the rotation of the engine. • I see no codes, but I’m seeing a bunch of different units when trying to see TDC. I never get a signal for desired/actual timing. Maybe cause batteries were low.

Tools • GMTDScanTech on laptop • Quadstar temp fooler • USB 12-pin OBD1 cable from OBD Diagnostics Inc.

Ask
At this point the truck starts, scan tool talks, no codes, but it runs rough and smokes heavily. If anyone has questions that would help point me in the right direction, let me know and I’ll test/answer.