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240 Idle issues and wonky vacuum gauge reading

Fjergus

Fjergus

New member
Joined
Dec 5, 2014
Location
Seattle-ish
1985 240DL with a freshly rebuilt NA B230 and a K cam (about 10k miles on the rebuild). The engine is out of a later 240 (a 1993, if I recall correctly). LH2.2 ECU, Chrysler ignition, and an M46.

I swapped my tired old engine out for a freshly rebuilt one last year, and the damn thing has never really run properly. It seems to behave itself at higher RPM, but it runs very rough at idle. It behaves a bit like a really bad vacuum leak, but I have spent many, many hours searching for one in vain. No leaks around the intake manifold gasket, no leaks in the intake boot or any of the hoses, and the air fuel mixture stays pretty steady at 14.7 (I installed a wide band sensor a while back for diagnostic purposes). It just sputters and shakes like a dog ****ting a peach pit anywhere below about 2000 RPM. It also runs slightly better for the 30 seconds or so when I start it cold, but it starts to run like **** almost immediately.

I'm fairly confident the IAC is working correctly: turning the AC knob causes the idle to increase by a few hundred RPM, and unplugging the IAC at idle causes the idle speed to drop slightly.

The ignition timing is set correctly according to the book. It is worth noting that the timing wanders around a ton at idle, but that's pretty normal for the Chrysler ignition systems from what I understand.

Valve timing was set correctly when I put it together, and it hasn't changed since then.

I recently got a vacuum gauge:
  • At idle, the reading rapidly oscillates between roughly 10in-Hg and 15in-Hg. The oscillation appears to match the engine RPM.
  • At roughly 1700RPM, the vacuum bounces from around 16in-Hg to 20in-H and back again, completing a cycle roughly once per second. The needle stays steady between cycles (it doesn't jiggle like it does at idle).
  • At steady state throttle anywhere else in the rev range, the vacuum stays steady around 20in-Hg.

Any ideas? According to various sources on the intertubes, the jiggly idle vacuum business may be caused by worn valve guides (or maybe just the wrong ones, since this engine only has about 10k miles on it), or some other valve train issue. I'd be pretty damn irritated if that were the case (again, only 10k miles), but that's kinda the direction I'm leaning. Thoughts?
 
The idle you are experiencing is probably normal for an engine that is using a K cam. Another thought with that timing wandering you mentioned. That isn't normal in my experience. It could be the crankshaft pulley is slipping. That is one of symptoms of that happening.
 
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Please post a video of the idle and higher rpm scenarios you mentioned. Like dl242gt said it may be just the rough cam. I cannot get my b-cam to idle smooth enough for my liking until 900-950rpm, and the k-cam has much more lift and duration.

My idle oscillates from 15-18mmhg when warm @900rpm, and higher RPM it stays much more steady
 
With a mild street performance cam like a enem V15. A B230 is not going to idle like a M or even the T cam. The K cam has about another 20 degrees of duration compared to my V15 or a B cam which both have 256 degrees duration. It might help to advance the cam some. I use 6 degrees advanced with the V15.
 
I'll post a video tomorrow, and possibly do a compression test to figure out if I have a leaky valve on one cylinder or something like that. I don't really think this is just normal lumpy cam behavior. The K cam was the standard in Canada (if I remember correctly), and I can't imagine them selling cars that idled this badly. Esmth, how rapidly does your oscillation happen at idle? The needle on mine seems to jiggle back and forth about once per engine rotation, from what I can see. That's what makes me think that one of the cylinders might be pulling less vacuum than the others.
 
I'll get a video for you tomorrow. But it oscillates pretty fast up and down ~1-3mmhg (depending on the weather) during idle. Even more on a cold start. Mine is advanced 4* so take that into consideration
 
Cam/distributor(pickup coil?)
Crankshaft pulley for timing
Vacuum leak check brake booster hose and hose to Chrysler crap box just in case if you didn't.
 
When I ran a K cam in my family's '85 245DL back in 2002, I set the idle around 850-900 rpm. Ignition timing at 7-8 deg BTDC. It ran very rough at 750, which is to be expected. Once it came on cam at 2000 rpm, it pulled strongly all the way to the 5800 rpm rev limiter. That said, I had a true B23E in an '81 242DL a few years ago (1999-2000 time period). With the K cam, it surprisingly ran nicely at 750 rpm and would run well at the 1989-on manual 240 idle speed of 540 rpm. So, YMMV.
 
Hokay, here's a video

It isn't easy to see in the video, but the whole car jiggles around pretty significantly at idle. It is significantly worse when the engine is warm. It's bad enough to be kinda uncomfortable if you're sitting in traffic for a while.

I also discovered a new symptom while I was filming this: the vacuum oscillations at idle gradually increase to the point where the needle is wildly flailing around between 10 and 17 in-Hg. Blipping the throttle seems to "reset" the oscillations, but they gradually creep upwards again after a few seconds. It only seems to do this when the engine is warm, interestingly. It also doesn't seem to coincide with an audible change to the engine noise, and the AFR doesn't do anything weird while that's happening.
 
Definitely do that compression test. That looks like one of the cylinders isn't holding up.

Water in oil? Excessive pressure in crankcase? Overflowing coolant reservoir?
 
Yep, I'll stop and grab a gauge on my way home tonight.

No sign of a leaky head gasket, luckily. No oil milkshake, exploding coolant, or farting crank case. I strongly suspect one of the valves isn't sealing properly. Here's hoping it's just a shimming issue, as opposed to a burned/bent/borked valve issue. Le sigh.
 
Fjergus said:
Hokay, here's a video

It isn't easy to see in the video, but the whole car jiggles around pretty significantly at idle. It is significantly worse when the engine is warm. It's bad enough to be kinda uncomfortable if you're sitting in traffic for a while.

I also discovered a new symptom while I was filming this: the vacuum oscillations at idle gradually increase to the point where the needle is wildly flailing around between 10 and 17 in-Hg. Blipping the throttle seems to "reset" the oscillations, but they gradually creep upwards again after a few seconds. It only seems to do this when the engine is warm, interestingly. It also doesn't seem to coincide with an audible change to the engine noise, and the AFR doesn't do anything weird while that's happening.
Click to expand...

that's way more oscillate-y than mine. i'm interested in the results of the compression test. During the 10% throttle part of your video where the car revs up and down on its own reminds me of what my car does when the idle valve gets sticky. i'd try cleaning it. that won't solve your other problems though
 
Yeah, I was wondering if the IAC might be causing that part. I'll try unplugging it and seeing if it goes away. Depending on the results of the compression test, I might pop the valve cover off and see what the clearance looks like. I'm thinking they might just have shimmed an exhaust valve incorrectly.
 
Get a set of the angled feeler gauges. They are a bit easier to insert to measure the clearance. I use .014, .015, .016" as a go no go or too loose to get them around .015" cold.
 
The idle seems normal compared to my engine, which should be a K-Cam from the vin - 1983 B23E K-Jet... If not a little smoother than mine.

I just put it in neutral at traffic lights when the car starts to shake due to my idle being at about ~700/800



As a note, can you plumb in a vacuum gauge into the A/C controls or something? would be cool to have a gauge in the dash.
 
Hmm, interesting development: all 4 cylinders pump out 180 psi on the dot. Soooo... any other ideas? I'm going to pull the IAC and the throttle body off this weekend and see if giving them a thorough scrubbing helps the weird off-idle wandering thing, but beyond that I'm pretty much back to scratching my head.

Also, the plugs all looked good when I had them out. Not that I was expecting to find any surprises in there, but still.
 
What is the valve lash set at? Try advancing the cam a bit maybe six degrees advanced?

After that it's probably what I wrote in my earlier post. I can't see the link to your video.
 
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