Trying to diagnose a possible vacuum related issue (Page 1/1)
SpeedyBark MAY 22, 09:34 PM
Heya folks! Having a little trouble right now, and looking for some direction. 1984 2.5L M/T, 180K or so miles, white with rust accents, and no reported diagnostic codes

It's hard to give a short version of the problem. I think I'm having a sensor issue, as the car doesn't want to start or stay running, but it will given some coaxing.

I drove it infrequently last summer after fixing the oil pan (thanks to you fine folks here!) It would always start and idle very high until it warmed up, then drop to a normal operating RPM and be fine. Towards the end of the summer, the engine would stutter during acceleration. This was annoying, but I figured something was just dirty given its age and state of care, so I ran some high mileage seafoam through it. This did not help. One early fall day, at a busy four way intersection, JUST outside of push-it-home distance, it decided to stutter for the last time, and stalled without restarting. One short conversation with the deputy about why the local police no longer have push bars later, it was towed a few hundred yards home. For whatever reason, like an absolute fool, I convinced myself it was a fuel pump problem and removed the fuel tank to check. In the process I replaced the fuel feed, return, and vapor lines since they were dangerously rusty and destroyed in the process. The fuel pump was not the problem.

Fast forward to today: After doing some further reading here, I verified that it wasn't an ignition problem. I decided to try some starting fluid, and lo, it happily ran until the magic spray ran out. Suspecting it might be an EGR issue, I squeezed the bottom of the diaphragm and tapped on it a bit, not really having a clue what I was doing nor trying to accomplish anything in particular. Reassembling everything and trying a bit more starting fluid and a little bit of throttle feathering, it finally kicked over, idled really high during warmup as it always does, then dropped to a nominal RPM and died the moment I gleefully skipped off to inform the world of my success. I was sort of able to restart it, but that's where things get really weird. If I sort of pump the gas pedal, the RPM will actually increase when I let off. I can pretty indefinitely keep it running, but only by pumping the pedal - it stalls out with no input and with the pedal held down.

I'm not really sure where to go next - I suspect it's something to do with the EGR since it just didn't run at all until I started prodding at that bit, but I don't really know enough about EGRs to figure what to do next. If the EGR is in fact the issue, I'd like to fix it, though I wouldn't be against blocking it. Honestly, I don't know what "blocking" it actually means as I haven't yet found a guide on how to do that. If it's not the EGR, then I'm not sure what it would be. Any insight y'all have is appreciated.

Thank you very much!
Gall757 MAY 22, 10:02 PM
The starting fluid successfully running the engine tells you that something is wrong with your fuel system.....either low pressure or a faulty injector. Change your fuel filter just to make sure that it is not the problem. Getting fuel pressure on a Duke is not easy, but there are threads here describing how to do it. The Ogre's Cave has a TBI adapter for fuel pressure testing.

[This message has been edited by Gall757 (edited 05-22-2022).]

SpeedyBark MAY 22, 10:27 PM
That is entirely possible, if not probable, but I do want to clarify that it ran happily for several minutes while warming up with no starting fluid. Further, I can run it at a high RPM using the wacky let-off procedure I described, again, with no starting fluid. Would that sort of behavior be observed with low fuel feed pressure?

Thank you!
theogre MAY 22, 10:38 PM
See my Cave, Vac Leaks

Blocking EGR... besides Illegal can cause problems because ECM expect it to work and Fuel and Ign maps are programed of working EGR. Even when EGR has no connection w/ ECM.

ECM could be "dead." Example:
ECM must see working Ignition before "turn on" Injection...
Ignition and wiring can have problem while still making spark. See my Cave, HE Ignition & Ground "Myth" notes
Or ECM "see" ignition working and tries to turn on injection but Injection section is "bad."

Can have problems w/ Injector, IAC, and more.
IAC does more than just setting Idle. ECM uses as a "choke" to start the car.
See my Cave, Sensors Quick Ref

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

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