RS Disposal Posted November 18, 2025 Share Posted November 18, 2025 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 November 19, 2025 Share Posted November 19, 2025 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 November 19, 2025 Author Share Posted November 19, 2025 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 November 19, 2025 Share Posted November 19, 2025 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 November 19, 2025 Share Posted November 19, 2025 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 November 20, 2025 Share Posted November 20, 2025 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 November 21, 2025 Author Share Posted November 21, 2025 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 November 21, 2025 Author Share Posted November 21, 2025 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 November 21, 2025 Author Share Posted November 21, 2025 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 November 21, 2025 Share Posted November 21, 2025 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 November 21, 2025 Share Posted November 21, 2025 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 November 21, 2025 Author Share Posted November 21, 2025 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 November 21, 2025 Author Share Posted November 21, 2025 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 November 21, 2025 Share Posted November 21, 2025 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...
Popular Post RS Disposal Posted January 9 Author Popular Post Share Posted January 9 I want to thank everyone for your replies. Please excuse my terminology whereas I am a small truck/auto mechanic, not a heavy truck mechanic. I am just a one man show with only one truck and I do try to do most of my repairs\maintenance myself. That the way my dad did it and taught me.. I may of found my problem, if not it was a problem only going to get worse. During some routine work I noticed a fuel line along side the frame rail going from what I assume is a fuel pump on the side of the injector pump the was wet and the under side of the truck and leaf spring also wet. My truck has 2 fuel filters. primary has the line from tank then goes to this fuel pump. From there the line goes to the secondary fuel filter and from there to the injector pump. I am assuming these last two lines are under pressure hench the reason the line is wet from leaking. The other lines are suction lines. I noticed that last time I replaced fuel filters that the secondary was only about 80% full. The line that was wet was the one from the pump to the secondary filter. So my thinking is that for whatever reason pressure was being lost and air introduced into the system. Replaced the two lines off the secondary filter and I guess time will tell if that was my problem. As soon as the weather gets a little better will be replacing the other fuel lines. Don't have a shop big enough to get truck inside so it is all outside work.😪 2 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-588756 Share on other sites More sharing options...
Popular Post terry Posted January 9 Popular Post Share Posted January 9 If it is leaking fuel out, it is probably letting air in, terry 4 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-588769 Share on other sites More sharing options...
RS Disposal Posted 23 hours ago Author Share Posted 23 hours ago OK, an up date on my problem. The issue is now that when climbing the canyon occasionally the truck will start to miss to the point I have to pull over and let engine idle for a bit at which time I can continue on up. Replacing the in/out fuel lines from the secondary fuel filter didn't really make any change. I replaced the fuel line from primary filter to the fuel pump, again with no change. Still have to change out the line from tank to primary, but that one is alot more time consuming because of the way the factory routed it. Thinking I will route it in a more direct fashion from tank to filter rather than down along frame to behind trans and across and back up to filter. The line doesn't appear wet anywhere, but being on the suction side I am thinking it might not be. This line being under the truck is exposed to alot of weather and road spray/salt. Plus the truck is 47 yrs old. Not alot of miles but I do know rubber breaks down because of age. just trying to learn my system also and whether or not this could be a problem. The first pic is of my intake manifold with where I assume this senses boost pressure and sends a signal to the injector pump. (where my finger is pointing) The larger line is truck air and the smaller is the one that goes around to the injector pump. The third is on the side of the injector pump where the small line comes down from the row of injectors and the larger line goes back to the tank. On another note the couple of times I have replaced/taken the secondary fuel filter of it hasn't been completely full. Maybe about 80%. In my way of thinking it should be full. Would this be an indication the air is entering my fuel lines somewhere between tank and injector pump? Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-589191 Share on other sites More sharing options...
terry Posted 21 hours ago Share Posted 21 hours ago Have you checked the two plastic check valves in the fuel lift pump? terry Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-589196 Share on other sites More sharing options...
RS Disposal Posted 17 hours ago Author Share Posted 17 hours ago 4 hours ago, terry said: Have you checked the two plastic check valves in the fuel lift pump? terry not exactly sure where that is. I am not a heavy truck mechanic and still learning my truck all beit it is older Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-589220 Share on other sites More sharing options...
terry Posted 16 hours ago Share Posted 16 hours ago (edited) Right under the front of injection pump, has two fuel lines on the bottom of it, with two capscrews with copper washers on the top of it, take the nuts off and there is a plastic check valve under each one. You can see the top of it on the last pic. terry also a small spring on each one Edited 16 hours ago by terry 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-589226 Share on other sites More sharing options...
fjh Posted 2 hours ago Share Posted 2 hours ago 14 hours ago, terry said: Right under the front of injection pump, has two fuel lines on the bottom of it, with two capscrews with copper washers on the top of it, take the nuts off and there is a plastic check valve under each one. You can see the top of it on the last pic. terry also a small spring on each one Also check the return check Tee there if you take it apart to see if the spring is broken that can cause low pump pressure ! you need bare min 15 psi to run clean at full load fuel demand ! 2 Quote Link to comment https://www.bigmacktrucks.com/topic/76164-occasional-engine-surge/#findComment-589251 Share on other sites More sharing options...
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