Hey guys, I have an idle issue that I just can't figure out and I'm hoping someone can point me in the right direction. I just swapped a 5.9 and 46re into my jeep TJ. From the start it has had an idle surge and it dies after surging a few times. Upon startup, engine revs to 1500-1800rpms for a couple of seconds, then it comes down to 750-800 or so, surges to 1000 and then drops to 300 within a second or so. It does this a few times then dies. The initial rpm seems to be the IAC starting out at 140ish steps, the rpms come down as it closes to around 40 then it goes up and down like its trying to compensate for the idle rpms being too high or too low. I have the plenum sealed with an aluminum plate and right stuff using new grade 8 bolts. I smoke tested it for vacuum leaks twice. Ive capped off all the vacuum ports. Replaced pcv. I have new plugs in the engine, cleaned throttle body and all passages replaced IAC, replaced map sensor, replaced IAT sensor, replaced coolant temp sensor, replaced TPS, reset tps minimum with scanner, reset all adaptives with scanner, replaced both O2 sensors, cat isn't plugged. Engine has good grounds, both the factory ones in the donor harness and 2/0 negative cables to both block and starter bolt. I noticed fuel pressure fluttered on the gauge while the engine was surging, so I replaced the fuel pump. Pump pressure is 45-46 at key on, 49 running at rpm over 850 and still flutters between 42 and 49 when engine is surging. I set the fuel timing to 0° when I first started the engine since I'd had the distributor out when I resealed it. Engine has 140k miles on the short block, but it still looked pretty healthy. I put reman heads on it when I had it out since the originals had the typical cracks. Compression is 140ish or within 5% of that on all cylinders. I just can't figure out this idle issue. Engine runs pretty smooth when I hold the throttle open. Also will stay idling if I unplug the IAC before it has brought the rpm all the way down. The other thing, if I unplug the map sensor it will idle pretty steady at about 750-800rpm in map fault mode when monitoring with a scanner. Does anyone have any other ideas to try? I'm open to just about anything. Thanks

 

I would suspect you have a vacuum leak somewhere.

 

That is kinda what it seems like, but I sure can't find anything. I just got home from work, checked to make sure all the vacuum ports were capped and tried it again. Same result. Pulled the throttle body and checked the plenum plate seal with a flashlight and mirror, it still looks perfect, bright clean aluminum with no oil and a goos squeeze of right stuff all the way around it. I taped off the top of the manifold where the throttle body goes and put the smoke machine too it. I'm not getting smoke out anywhere. Throttle body gasket looked like an even crush all the way around. The little vacuum elbow on the map sensor looks good, no cracks or anything. Is there anywhere else could I be getting a vacuum leak from?

 

Assuming by all ports capped you don't have a brake boost or anything hooked up, I'm not sure if the swap still used vacuum for cruise or 4wd?

 

Mine was like this.
Start w/the check valve.
End w/the intake gaskets.

Have fun.
Good Luck

 

I have vacuum boost brakes on it still, but I pulled the hose and capped the port to eliminate that. Same for the line to the vacuum reservior and dash controls. Every port on the manifold is capped right now. Even the pcv hose. No cruise control as this is a rockcrawler build. I can't think of anywhere else it could be leaking from. I tried the TSB method for checking plenum leaks as well. Pinched off the pcv hose, put gauge on the breather and it's not pulling a vacuum in the crank case. Is there anything else that could be causing this situation?

 

If you put a vacuum gauge on one of the ports, and let the engine idle, what does the gauge do?

 

It goes up and down with the rpm. Gauge reads about 22" at the top of the surge, 1000rpm and drops to 18" at 700. Continues dropping with the rpm from there. Map sensor reads 20" when the gauge reads 22. I've tried 2 other map sensors and they all read the same. And they read the same tee'd into the same port as the gauge. So its possible my gauge is off by 2".

 

Vacuum

 

try the simple things, have you sprayed starting fluid around the keg, this can always always help finding a tricky vacuum leak, I know you smoke tested, sometimes the simple things work better

Its been a minute since I had a keg off but is it possible for the intake gaskets to leak from crankcase (lifter valley) into cylinder ports, say the keg was not fully seated due to something else