BullnoseLuvr Posted July 30, 2020 Share Posted July 30, 2020 So I've got all my engine sensors and controls and the EEC replaced or working (except for the Thermactor system -- coming). The engine harness was crumbling in my hands so I pulled it out and rebuilt it. On cold start the choke works, closing on a pump-and-a-half before start and opening slowly and fully on warm up and the vacuum exhaust/intake manifold heat valve closes. Cold idle to warm idle seems to work too. But, after just under 60 seconds after start the engine suddenly falls down under 500 hundred RPM and stumbles for a few seconds. It recovers and the exhaust heat valve opens and everything is fine. I plan to go through all the factory troubleshooting steps soon, but I'm curious if anyone has seen this and if there's a simple and common explanation. If the EGR valve opens on cold idle (I think) it can really wreck the idle. So maybe this is the problem? One of the VCV's? I think I replaced both of them, though it's been a while. Mark Link to comment Share on other sites More sharing options...
FuzzFace2 Posted July 31, 2020 Share Posted July 31, 2020 Easy enough to test, pull the vacuum line to the EGR and plug it and see if it stumbles. Dave ---- Link to comment Share on other sites More sharing options...
Gary Lewis Posted July 31, 2020 Share Posted July 31, 2020 It sounds like it is computer-controlled as I'm assuming you aren't moving the throttle during that time. Given that the fast idle shouldn't be coming off and dropping the idle as it isn't supposed to come off until you open the throttle slightly. But I'm not up on what all the computer does with its vacuum valves. Perhaps the diagnostic procedure will tell, but if not can you put your hands on some of the vacuum valves and tell if any open at just the right time to cause that? Link to comment Share on other sites More sharing options...
BullnoseLuvr Posted July 31, 2020 Author Share Posted July 31, 2020 Easy enough to test, pull the vacuum line to the EGR and plug it and see if it stumbles. Dave ---- I think the EGR is the likely culprit and I'm going to try that. I tried it again tonight and it was a bit cooler out. It took longer to get to the stumble (but of course I didn't time it this time) so it's probably temperature related and it took longer to get there because it was cooler. If it turns out it is the EGR, which has a new position sensor on it and I assume it is working right, then it's probably opening too soon. Some of the emissions controls are time related from start, so maybe if there is a timer element in addition to temperature to the EGR valve opening (which I think there is) maybe it's fubar. If defeating the EGR fixes the problem then I'll do a full diagnostic. Mark Link to comment Share on other sites More sharing options...
BullnoseLuvr Posted July 31, 2020 Author Share Posted July 31, 2020 It sounds like it is computer-controlled as I'm assuming you aren't moving the throttle during that time. Given that the fast idle shouldn't be coming off and dropping the idle as it isn't supposed to come off until you open the throttle slightly. But I'm not up on what all the computer does with its vacuum valves. Perhaps the diagnostic procedure will tell, but if not can you put your hands on some of the vacuum valves and tell if any open at just the right time to cause that? Correct. I'm not touching the throttle before the stumble for that reason -- to leave it on the cold start fast-idle notch. I tested the VCV's a few years ago and replaced one and I hope one hasn't failed since but I'll double check. Mark Link to comment Share on other sites More sharing options...
BullnoseLuvr Posted August 1, 2020 Author Share Posted August 1, 2020 Correct. I'm not touching the throttle before the stumble for that reason -- to leave it on the cold start fast-idle notch. I tested the VCV's a few years ago and replaced one and I hope one hasn't failed since but I'll double check. Mark I disconnected the EGR and there's no stumble. The heat riser valve still opens after what would be the moment the EGR kicks in. I'm pretty sure the EGR isn't supposed to ever open during idle. It appears this one opens for just about 5 seconds and closes again. Link to comment Share on other sites More sharing options...
FuzzFace2 Posted August 1, 2020 Share Posted August 1, 2020 I disconnected the EGR and there's no stumble. The heat riser valve still opens after what would be the moment the EGR kicks in. I'm pretty sure the EGR isn't supposed to ever open during idle. It appears this one opens for just about 5 seconds and closes again. Well there you go you now know where you have to look deeper into for a fix. Dave ---- Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 2, 2020 Share Posted August 2, 2020 I disconnected the EGR and there's no stumble. The heat riser valve still opens after what would be the moment the EGR kicks in. I'm pretty sure the EGR isn't supposed to ever open during idle. It appears this one opens for just about 5 seconds and closes again. I agree - you shouldn't have EGR at idle. Only at cruise with high vacuum. Link to comment Share on other sites More sharing options...
BullnoseLuvr Posted August 2, 2020 Author Share Posted August 2, 2020 I agree - you shouldn't have EGR at idle. Only at cruise with high vacuum. If I remember correctly (and I'll look it up). The amount of vacuum doesn't matter on the EEC-IV system -- the EGR valve is controlled by a VCV and the computer. On earlier systems there was what I think is a feedback system or something like that, which was dependent on the level of vacuum for proper operation. Sort of. Mark Link to comment Share on other sites More sharing options...
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