RS Disposal Posted Tuesday at 10:00 PM Share Posted Tuesday at 10:00 PM I have a 1979 ENDT 675 Mack engine. Recently it has developed a surge at about 1500 rpm's unless the engine is under a load. Nothing around 1200 or upwards of about 1900. When it does surge the boost gauge remains steady. I don't think it is throttle linkage, but I could be wrong. Haven't tilted the cab to really look at it yet. It is definitely rpm related and not truck speed related. Any ideas as to where to start? You guys were awesome help a couple years back when the engine dropped an exhaust valve in helping me source parts and do the repair. nobody in my area wanted to work on this old truck and I am a light truck mechanic so it was definitely a learning experience for me. TIA Rob Shane RS Disposal LLC Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/ Share on other sites More sharing options...
fjh Posted Wednesday at 06:03 PM Share Posted Wednesday at 06:03 PM can you identify the fuel pump for us! Robert bosch or ABAC? Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585793 Share on other sites More sharing options...
RS Disposal Posted Wednesday at 08:44 PM Author Share Posted Wednesday at 08:44 PM 2 hours ago, fjh said: can you identify the fuel pump for us! Robert bosch or ABAC? I csn try and look at it when I get to the truck tonite. Not sure how I would tell. What should I be looking for as far as the difference. Like I have stated I'm not really a heavy diesel mechanic. I can take a pic and upload it Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585813 Share on other sites More sharing options...
D-Day Posted Wednesday at 10:15 PM Share Posted Wednesday at 10:15 PM There should be a tag riveted to the side of the pump that will tell you what pump you have and the pump number Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585821 Share on other sites More sharing options...
fjh Posted Wednesday at 11:42 PM Share Posted Wednesday at 11:42 PM On 11/19/2025 at 12:44 PM, RS Disposal said: I can take a pic and upload it I can take a pic and upload it do so please! I’m not a numbers guy! In an Ambac Pump there is an over speed clutch that can start slipping! And this can lead to a run away! Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585827 Share on other sites More sharing options...
Full Floater Posted Thursday at 03:15 PM Share Posted Thursday at 03:15 PM Any chance that it's sucking air into the fuel system, or fuel restriction? Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585860 Share on other sites More sharing options...
RS Disposal Posted 8 hours ago Author Share Posted 8 hours ago sorry about the dirty looking fuel pump. Best pics I could get being the pump is down alongside the frame rail Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585915 Share on other sites More sharing options...
RS Disposal Posted 8 hours ago Author Share Posted 8 hours ago On 11/19/2025 at 3:15 PM, D-Day said: There should be a tag riveted to the side of the pump that will tell you what pump you have and the pump number This was the only tag I could find, but the pump was pretty dirty being the truck works in a very dusty enviroment Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585916 Share on other sites More sharing options...
RS Disposal Posted 8 hours ago Author Share Posted 8 hours ago On 11/20/2025 at 8:15 AM, Full Floater said: Any chance that it's sucking air into the fuel system, or fuel restriction? That was on thing I was thinking might be, a fuel restriction. When I changed fuel filters, thinking it was plugged filters, and I cut one open there was a black residue in the filters. My thought weas that maybe the fuel lines were beginning to deteriorate internally being the truck is 1979 (46 yrs old) I have purchased new fuel line, just haven't replaced it yet Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585917 Share on other sites More sharing options...
fjh Posted 8 hours ago Share Posted 8 hours ago Give it a try definitely worth replacing them ! The pump is Ambac worth keeping an ear on too the over run clutch can be an issue! Snap the throttle and listen for it jumping to governed rpm if it travels beyond the governed by more than 50 you may need to have the drive hub serviced! Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585920 Share on other sites More sharing options...
The Heinz Posted 7 hours ago Share Posted 7 hours ago What's your fuel tank look like? You could have gotten some bad fuel or something if there's residue in the tank. We've seen it happen locally around here; an entire fleet was having some fuel issues (all newer trucks), and it turned out to be a batch of bad fuel at the local fuel station all their drivers were going. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585921 Share on other sites More sharing options...
RS Disposal Posted 5 hours ago Author Share Posted 5 hours ago 2 hours ago, fjh said: Give it a try definitely worth replacing them ! The pump is Ambac worth keeping an ear on too the over run clutch can be an issue! Snap the throttle and listen for it jumping to governed rpm if it travels beyond the governed by more than 50 you may need to have the drive hub serviced! when I snap the throttle the rpm's don't run up. It immediately drops back down to idle. Truck doesn't have alot of miles, only 160,000. It's just old but other than the occasional surge it runs and pulls great Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585924 Share on other sites More sharing options...
RS Disposal Posted 5 hours ago Author Share Posted 5 hours ago 2 hours ago, The Heinz said: What's your fuel tank look like? You could have gotten some bad fuel or something if there's residue in the tank. We've seen it happen locally around here; an entire fleet was having some fuel issues (all newer trucks), and it turned out to be a batch of bad fuel at the local fuel station all their drivers were going. I believe the fuel tank is good. when I did run it lower on fuel I did shine a light into it and it looked clean 2 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585925 Share on other sites More sharing options...
fjh Posted 3 hours ago Share Posted 3 hours ago Go with your line theroy and see how it goes ! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-585932 Share on other sites More sharing options...
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