DirtWorx Posted January 25 Share Posted January 25 1999 Mack RD688S E7 misfire under load, cuts out sporadically - injectors, EUP, ECM, or wiring harness? Engine runs correct under normal operating conditions unloaded. Engine cuts out sporadically for a split second then comes right back on when loaded or unloaded. Engine sputters under uphill load until throttle is decreased. Engine injector(s) disengages and reengages sporadically under uphill load. Diagnostics shows injector 2 and 3 low voltage. Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/ Share on other sites More sharing options...
fjh Posted January 25 Share Posted January 25 Possible Fuel starvation? How’s the fuel pressure! Check you fir tank vent for starters! Check and clean the tack pu on the flywheel housing ! Clean the grounds below the starter all good first looks! Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545590 Share on other sites More sharing options...
Joey Mack Posted January 25 Share Posted January 25 check the ground wires at the big bolt on the block in front of the starter.. Is the coolant low, below the sensor? does the shutdown light come on? it seems like the issue is when going uphill.. I wouldnt throw any parts at it yet. I bet its a loose/bad wire. yes, check fuel pressure, should be around 80 PSI. Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545591 Share on other sites More sharing options...
JoeH Posted January 26 Share Posted January 26 Sounds like an issue with your #2 and #3 EUP's, anyone able to elaborate on what to inspect? I'd start with inspecting the wiring harness for any rub through, and make sure those 2 injector screws are tight. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545607 Share on other sites More sharing options...
JoeH Posted January 26 Share Posted January 26 Your point #2, is the engine shutting off momentarily? This will likely be a loose fuse for the EECU, I'm guessing this fuse is in the fuse box under the hood. With engine running, wiggle the EECU fuse. It'll probably stall out. It should trip a blink code 9-2 I believe. If your fuses aren't labelled in the box lid then just wiggle the fuses one by one. You'll need to pinch the fuse slot connections a bit tighter, and maybe bend the fuse legs a bit to create more connection. That's what I had to do on my MR688. Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545608 Share on other sites More sharing options...
Joey Mack Posted January 26 Share Posted January 26 the EECU fuse is #40 in the box under the hood. 9-2 code is '' power reset without the key'' I lean towards a loose ground. yes, a 'wiggle' wire/fuse test may just point you to the issue.. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545609 Share on other sites More sharing options...
JoeH Posted January 27 Share Posted January 27 On 1/25/2024 at 3:41 PM, DirtWorx said: 1999 Mack RD688S E7 misfire under load, cuts out sporadically - injectors, EUP, ECM, or wiring harness? Engine runs correct under normal operating conditions unloaded. Engine cuts out sporadically for a split second then comes right back on when loaded or unloaded. Engine sputters under uphill load until throttle is decreased. Engine injector(s) disengages and reengages sporadically under uphill load. Diagnostics shows injector 2 and 3 low voltage. Find a solution yet? Quote Link to comment https://www.bigmacktrucks.com/topic/73786-1999-mack-rd688s-e7-misfire-under-load-cuts-out-sporadically-injectors-eup-ecm-or-wiring-harness/#findComment-545662 Share on other sites More sharing options...
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