Old school breathing faults

Geoff Diagnostician Lahaina, Hawaii Posted   Latest   Edited  
Question
Driveability
2003 Chrysler PT Cruiser GT 2.4L (G) 4-spd (41TE)
Rough Idle

This is really not vehicle specific, but I listed the vehicle I was specifically looking at. Everybody in this area has limited funds and barely "needs" their car, so many times they will not dig deeper on driveability, and I am left wondering "wtf was wrong with that?". (Cars usually have many many issues too. )

This question is likely best suited for the guys who grew up before sensors and waveforms . Every test I ran was with a gauge. That's what I have, and that's really all that was needed back in the day, when guys had a lot more experience tearing engines apart. Only so many things can go wrong, right?

For the sake of the discussion/theory you must assume fuel delivery and ignition are 100% perfect.

So...what causes all this on a non-VVT DOHC 4-cyl:

1. The engine has near 0 psi back-pressure at 2000 RPM but the gauge bounces like ffffing crazy at idle.

2. The engine vacuum is 20 in-Hg at 2000 RPM but only 15" at idle. (both steady, not bouncing)

3. Compression is 160-170 on all cylinders with WOT and fuel/ign disabled and battery charger

4. All cylinders have only 5% leak-down

5. All timing marks line-up and dowel-pins do not appear sheared.

6 . Cranking vacuum is shown in this video: (sorry for the double start)

youtu​.​be/7zptVX4pfCo

I wanted to attach the MP4 but I can't

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Cuba Mobile Technician
Phila, Pennsylvania
Cuba Default
 

IMO, 1 & 2 seem to point out the engine not breathing the most efficient way possible. Could prove valves distorted or not 100% sealing. 3, 4, & 5 are not 100% definitive in proving that engine internals are sound and free from defect. You can still have bent rods or valves but still pass compression & leak down tests. This is what I've gathered recently, I could be wrong. The most

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Geoff Diagnostician
Lahaina, Hawaii
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Thanks Cuba. I'm hoping someone that has experimented with moving cams around can just explain what happens when each one moves each way. Maybe a cam twisted or something. We almost never tear into engines here since they are just too old. The whole situation basically made me quite aware of something that I have never fully understood or maybe never even knew.

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Leo Technician
Rhinebeck, New York
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Your exhaust back pressure reading bounces like crazy at idle? But it is zero psi at 2000 RPM? Just making sure I fully understand your first parameter. If so, perhaps the exhaust system is full of water. Idle speed airflow could be less capable of keeping the water pushed out of its way, and would have to keep "burping" through the system.

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Geoff Diagnostician
Lahaina, Hawaii
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Interesting idea! Thanks Leo.

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Bruce Mobile Technician
Avon, New York
Bruce Default
 

that auto should run great !1 what's the specific Complaint ..???????????

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Cuba Mobile Technician
Phila, Pennsylvania
Cuba Default
 

Rough idle.

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Bruce Mobile Technician
Avon, New York
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set the idle rpm higher .. :)

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Cuba Mobile Technician
Phila, Pennsylvania
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Lol. Of course! 😉 I think OP just wants to know if the main issue can be pinpointed using the basic tools. I say, it can be narrowed but not be definitive.

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Bruce Mobile Technician
Avon, New York
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sorry, cuba.. I was trying to engage the OP on this my thoughts-- - Yep, the mechanical gauge will bounce at idle, after all there is not the continuous flow of gas that there would be at 2000 rpm - Yep, vacuum drops as throttle position increases - Yep, good compression - Yep, 5% L.D. is a decent figure, not knowing when it was measured - Yep, I could care less about "cranking

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Geoff Diagnostician
Lahaina, Hawaii
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Thanks for the reply Bruce. So you are saying that a back-pressure gage bouncing like mad at idle is not indicative of an issue? I thought it was telling me something and had no idea what.

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Allan Instructor
Winnipeg, Manitoba
Allan Default
 

Hey Geoff does this thing have an EGR valve? I try to avoid Chrysler products like the plaque so not sure if it does or not.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

It did not.

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Scott Owner/Technician
Brantford, Ontario
Scott Default
 

I would double check the cam timing, Is there a pid for cam /crank degree? I know you say it's good. 15 IN of vac to low. Dancing on cranking ,possible valve overlap.(see timing) Speed Density, low vac , skews the fuel map...

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Yes, there was "CMP CKP Offset " in the Data List and it was 1 degree, which would mean a lightly aged belt, but not a jumped tooth. I pulled the covers and looked anyway. It was suggested elsewhere that the cams can shear the pins and not stay aligned with the pulleys. Seemed a little far fetched that it would just move a little bit. (only enough to confuse the crap out of me...LOL) Do you

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Eric Owner/Technician
Edgerton, Wisconsin
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Geoff, I would say the camshaft timing is off on the exhaust camshaft. Here is my reasoning, then we will fit it to your list of 6 items to see if it fits. If the exhaust camshaft timing is off in the advanced position, meaning the exhaust valves will open sooner then the valves will open before the piston reaches bottom dead center, this will result in a loss of power and maybe some popping

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Thanks Eric. I knew somebody had to know this stuff. From your explanation, a retarded camshaft would certainly fit, and it is quite plausible since sticky things drag, and dragging behind would leave it retarded. I originally took the time to pull the cam sprocket bolts out, and I could see the pins still stuck in the cams, but I suppose it could have been an optical illusion. I held a

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