I apologize in advance for the title, but I have such a weird issue I don't know how to describe it. I bought a 98 Suburban K1500 with the 6.5L turbo diesel back in August as a project/collectors vehicle and have been slowly trying to diagnose this issue and feel like I've hit a brick wall. It currently has about 288,000 miles on it and it seems to have been well taken care of. From what I can tell, even though this is my first diesel, it runs very well and it doesn't seem to have any major mechanical issues. After purchasing it, I drove it over 650 miles home and this issue occurred. After getting some history about the vehicle while chatting with the previous owner, he said he had replaced the alternator twice fairly recently (he had a shop do it the second time cause he wasn't sure it was really the alternator, they claimed it was bad). So while on my way home, I noticed that the alternator would drop out intermittently and I became concerned the alternator was failing again based on our discussion. I stopped at an auto parts store on the way home, bought a replacement alternator, changed it out in the parking lot, and went on my way. Things improved, but it did not resolve the issue. The RPM gauge connector on the back of the alternator was older and damaged though, so I replaced that too once I got it home. Upon inspecting the wiring at home, I noticed several connections that were less than acceptable, so I re-terminated the connections and again noticed a slight improvement but not much.
After driving it shorter distances at home (50-100 mile trips) I began to notice that it wasn't just electrical. When the issue occurs, or at least when it's very noticeable, the RPMs drop to 0 even though the engine is running very well, the power steering doesn't seem to work (noticed in a parking lot), the AC doesn't get as cold as it does when the engine is cold, and the alternator seems to quit producing voltage and the battery light comes on flickering (as designed if the alternator is not outputting enough voltage, hence my theory it's not electrical anymore). When the engine is started from cold, it doesn't necessarily seem to have this issue, but once things start to warm up it starts occurring more and more. Engine temps seem to keep being ok and don't fluctuate much, but with the other issues I'm concerned the water pump is not actually getting turned enough but I have no way to verify that other than it hasn't overheated and the temps never get anywhere near 210.
I've replaced the serpentine belt, thinking that maybe there was an issue with the belt, no improvement. The belt tensioner seems to be over tightening the belt, based on another users experience on these forums, so that's probably something I will have to work on as well, but it doesn't make sense to me what I'm experiencing is due to the belt being overtightened. The power steering pulley moves freely, the ac compressor was replaced less than 60,000 miles ago according to his service records and seems to work without issue, the alternator has less than 2000 miles on it, the fan and water pump move freely, the tensioner pulley moves freely, but the vacuum pulley doesn't move freely and snaps back regardless of the direction I tried to turn it (I did not try hard, as I'm not knowledgable about it, just a note). I have not checked the crank pulley or the harmonic balancer for issues, that will be next on the list. I bought the 1998 service manuals and from what I can tell, everything I've checked in there doesn't point me in a direction of an issue, and the belt and pulleys are all the correct sizes and in the correct positions. I've attached a link to a YouTube video of the gauges, as well as the diagnostic checklist below. Hoping someone has experienced this before or has ideas of things to look at cause I'm at a loss. Again, I can't stress enough that the engine itself is running like a champ, no smoke, sounds good, and doesn't fluctuate when this occurs. Seems to just be issues with the belt components not turning enough or at all intermittently. I believe I've included everything I can think of. Thank you for taking the time to read this.
Gauge Behavior While Issue Occurs (YouTube) - (Link was restricted for first post, will followup in another post below with link to YouTube videos of issue)
![Image]()
Gauges when issue is not occurring, all seems to look normal.
2. 1998 K1500 Suburban with the 6.5L Turbo Diesel
3. 288,000 miles
4. Have not verified the model number of the injection pump.
5. K1500, but has the 2500 chasis as offered from GM.
6. Do you have an EGR on the engine? Mine has an F in the VIN number.
7. Air Filter condition (visual check). Aftermarket setup, filter looks ok.
8. Fuel filter condition (freshly changed, mileage since changed). Haven't put together a spreadsheet of the current maintenance items status, but have tons of service records. Can post if someone believes it's necessary.
9. Location of PMD/FSD? Relocated to drivers side by the fuse box in the engine bay. Was originally relocated behind the headlight, but another kit was installed and that's the one by the fuse box.
9a. If remote mounted, describe wiring harness (homemade or purchased from which vendor). Looks to have been purchased from vendor, but I don't have any documentation on that from what I have seen so far and have not looked it over in detail.
9b. Indicate the location and condition of the FSD/IP grounding wire. Have not looked for this, but believe this is unrelated.
10. Outside Temperature (C or F). _ °. Northern Alabama, 75 to 100 degrees during the day.
11. Service Engine Light while running?-Off, has never once come on.
11a. Service Engine light does glow during start/cranking/bulb check: Yes
12.Have you scanned for engine codes? No
13. Condition of Battery terminals: Cleaned/re-terminated and tightened.
14. Known condition and age of Batteries. 2-3 years old, seem to work just fine and have had no issues cranking or starting the engine.
14a. Are batteries a matched set of same age? Yes
15. Condition of Major Grounds (removed, cleaned and tightened) Have not verified all grounds. Cleaned one in the engine bay and then gave up on them after I started noticing mechanical issues too.
15a Have the batteries been individually load tested? No
16. Does engine crank, or "turn over"? Yes
16a. Does engine start and run? Yes
18. Does your Wait To Start light come on? Yes
18a.Number of seconds WTS light is lit: 5-10 seconds, haven't timed it, but notice that after the engine starts it flashes some times intermittently.
19. Engine Cranking speed (if you have an accurate tachometer). Haven't checked
20. Are you experiencing Stalling? No
21. Check turbo inlet and air filter for obstructions. Doesn't seem related, haven't had issues.
23. Lift pump test - Describe results. Will perform at some point, but not on my priority list.
24. Upon cold start, does the radiator hose get hard quickly? Haven't checked
25. Upon cold start, do you have excessive white smoke? No
26. Do you have excessive cranking time before the engine starts? Intermittently, but only after engine is warm on hot days. Most of the time no
27. Have you used the block heater? Does it affect engine starting? (only try for starting problems). No, haven't had during cold months.
28. Are all glow plugs in proper working order? I assume so, they were replaced a while back according to the previous owner.
29. During hard acceleration, do you have excessive black smoke? No
30. Do you have any unusual exhaust smoke issues? No
31. Turbo check out - Pass/Fail: Have not tested, but noticed I get intermittent readings on aftermarket gauge 2 owners ago installed. Don't believe it's very accurate.
32. Indicate fuel that you are using: I assume #2 Diesel from Shell or Buc-ee's.
33. Are you using any fuel additives? Seafoam, noticed turbo seemed to respond more after about 50 miles or so after adding Seafoam, but haven't had the vehicle long enough to say it had any impact.
34. Upon unscrewing fuel cap, do you have a large vacuum formed in the tank? I wouldn't say a large vacuum, but definitely a vacuum.
35. Do you have any service history available that might pertain to the problem you are having? Other than the alternator replacements, I haven't found anything else in the service records yet for recent work.
36. Please indicate any modifications to the vehicle that might help us diagnose better: Aftermarket intake, PMD relocation, aftermarket gauges (boost, trans temp, and exhaust temp (but it's not hooked up)). Previous owner said he adjusted the turbo slightly for increased boost (his words), but I've never seen the gauge read above 7 psi and haven't adjusted. Most of the time it reads 0 unless accelerating onto the interstate.
After driving it shorter distances at home (50-100 mile trips) I began to notice that it wasn't just electrical. When the issue occurs, or at least when it's very noticeable, the RPMs drop to 0 even though the engine is running very well, the power steering doesn't seem to work (noticed in a parking lot), the AC doesn't get as cold as it does when the engine is cold, and the alternator seems to quit producing voltage and the battery light comes on flickering (as designed if the alternator is not outputting enough voltage, hence my theory it's not electrical anymore). When the engine is started from cold, it doesn't necessarily seem to have this issue, but once things start to warm up it starts occurring more and more. Engine temps seem to keep being ok and don't fluctuate much, but with the other issues I'm concerned the water pump is not actually getting turned enough but I have no way to verify that other than it hasn't overheated and the temps never get anywhere near 210.
I've replaced the serpentine belt, thinking that maybe there was an issue with the belt, no improvement. The belt tensioner seems to be over tightening the belt, based on another users experience on these forums, so that's probably something I will have to work on as well, but it doesn't make sense to me what I'm experiencing is due to the belt being overtightened. The power steering pulley moves freely, the ac compressor was replaced less than 60,000 miles ago according to his service records and seems to work without issue, the alternator has less than 2000 miles on it, the fan and water pump move freely, the tensioner pulley moves freely, but the vacuum pulley doesn't move freely and snaps back regardless of the direction I tried to turn it (I did not try hard, as I'm not knowledgable about it, just a note). I have not checked the crank pulley or the harmonic balancer for issues, that will be next on the list. I bought the 1998 service manuals and from what I can tell, everything I've checked in there doesn't point me in a direction of an issue, and the belt and pulleys are all the correct sizes and in the correct positions. I've attached a link to a YouTube video of the gauges, as well as the diagnostic checklist below. Hoping someone has experienced this before or has ideas of things to look at cause I'm at a loss. Again, I can't stress enough that the engine itself is running like a champ, no smoke, sounds good, and doesn't fluctuate when this occurs. Seems to just be issues with the belt components not turning enough or at all intermittently. I believe I've included everything I can think of. Thank you for taking the time to read this.
Gauge Behavior While Issue Occurs (YouTube) - (Link was restricted for first post, will followup in another post below with link to YouTube videos of issue)
Gauges when issue is not occurring, all seems to look normal.
2. 1998 K1500 Suburban with the 6.5L Turbo Diesel
3. 288,000 miles
4. Have not verified the model number of the injection pump.
5. K1500, but has the 2500 chasis as offered from GM.
6. Do you have an EGR on the engine? Mine has an F in the VIN number.
7. Air Filter condition (visual check). Aftermarket setup, filter looks ok.
8. Fuel filter condition (freshly changed, mileage since changed). Haven't put together a spreadsheet of the current maintenance items status, but have tons of service records. Can post if someone believes it's necessary.
9. Location of PMD/FSD? Relocated to drivers side by the fuse box in the engine bay. Was originally relocated behind the headlight, but another kit was installed and that's the one by the fuse box.
9a. If remote mounted, describe wiring harness (homemade or purchased from which vendor). Looks to have been purchased from vendor, but I don't have any documentation on that from what I have seen so far and have not looked it over in detail.
9b. Indicate the location and condition of the FSD/IP grounding wire. Have not looked for this, but believe this is unrelated.
10. Outside Temperature (C or F). _ °. Northern Alabama, 75 to 100 degrees during the day.
11. Service Engine Light while running?-Off, has never once come on.
11a. Service Engine light does glow during start/cranking/bulb check: Yes
12.Have you scanned for engine codes? No
13. Condition of Battery terminals: Cleaned/re-terminated and tightened.
14. Known condition and age of Batteries. 2-3 years old, seem to work just fine and have had no issues cranking or starting the engine.
14a. Are batteries a matched set of same age? Yes
15. Condition of Major Grounds (removed, cleaned and tightened) Have not verified all grounds. Cleaned one in the engine bay and then gave up on them after I started noticing mechanical issues too.
15a Have the batteries been individually load tested? No
16. Does engine crank, or "turn over"? Yes
16a. Does engine start and run? Yes
18. Does your Wait To Start light come on? Yes
18a.Number of seconds WTS light is lit: 5-10 seconds, haven't timed it, but notice that after the engine starts it flashes some times intermittently.
19. Engine Cranking speed (if you have an accurate tachometer). Haven't checked
20. Are you experiencing Stalling? No
21. Check turbo inlet and air filter for obstructions. Doesn't seem related, haven't had issues.
23. Lift pump test - Describe results. Will perform at some point, but not on my priority list.
24. Upon cold start, does the radiator hose get hard quickly? Haven't checked
25. Upon cold start, do you have excessive white smoke? No
26. Do you have excessive cranking time before the engine starts? Intermittently, but only after engine is warm on hot days. Most of the time no
27. Have you used the block heater? Does it affect engine starting? (only try for starting problems). No, haven't had during cold months.
28. Are all glow plugs in proper working order? I assume so, they were replaced a while back according to the previous owner.
29. During hard acceleration, do you have excessive black smoke? No
30. Do you have any unusual exhaust smoke issues? No
31. Turbo check out - Pass/Fail: Have not tested, but noticed I get intermittent readings on aftermarket gauge 2 owners ago installed. Don't believe it's very accurate.
32. Indicate fuel that you are using: I assume #2 Diesel from Shell or Buc-ee's.
33. Are you using any fuel additives? Seafoam, noticed turbo seemed to respond more after about 50 miles or so after adding Seafoam, but haven't had the vehicle long enough to say it had any impact.
34. Upon unscrewing fuel cap, do you have a large vacuum formed in the tank? I wouldn't say a large vacuum, but definitely a vacuum.
35. Do you have any service history available that might pertain to the problem you are having? Other than the alternator replacements, I haven't found anything else in the service records yet for recent work.
36. Please indicate any modifications to the vehicle that might help us diagnose better: Aftermarket intake, PMD relocation, aftermarket gauges (boost, trans temp, and exhaust temp (but it's not hooked up)). Previous owner said he adjusted the turbo slightly for increased boost (his words), but I've never seen the gauge read above 7 psi and haven't adjusted. Most of the time it reads 0 unless accelerating onto the interstate.
