Greetings all, I've resurrected my Dodge after a 2+ year hiatus and am starting to schedule more work for it. After getting the transmission rebuilt, I most recently replaced the cat after driving about 400 miles with it in a severely plugged state. For this reason I am afraid that I might have either cracked my new-ish cylinder head, melted my injectors, or blown out a portion of my intake manifold.

Yesterday evening I ran a vacuum gauge from the passenger intake port and monitored it at idle. At start up it read 18 inches before dropping to a very steady 17" once the engine warmed up. I also had a Harbor Freight OBD reader that I use mainly as a toy (and to read DTCs) to monitor the engine at idle, and about 675 rpm it read my MAP as 12.4 inHg. Spark advance was 4 degrees ATDC, and both O2 sensors were receiving voltage, between 0.8 and 0.9V.

I'm not entirely happy with these vacuum readings--they seem low to me--but I also wanted to know that if with 1.7 roller rockers and a larger than stock throttle body (52mm) wouldn't I be drawing more air in as a result and thus have lower vacuum readings? As well, the stock exhaust manifold was replaced with mellow Gibson headers and dual cat-back mufflers, but I just don't know if by "encouraging" air flow any of the mods would reduce vacuum readings...

The plenum pan is from a Hughes kit that was re-installed maybe 25,000 miles ago and inspection through the throttle body led me to believe that it is still nice and shiny. No detonation or pinging.

There is no discernable loss of coolant, zero overheating, nor is there the stumbling idle of the type that would lead me to believe my cylinder head cracked, which is how I eventually discovered the last head problem.

I have ordered a set of Autolite 3923 plugs to replace the stock copper Autolites I currently run; the 3923's are supposed to run "cooler", and with the Hemifever tuner it has been suggested I try them. And with phenomenal gas prices I had been driving in 93 octane mode this spring, which I guess could make for hotter combustion temperatures. I will check the originals when I pull them and report back, particularly as they relate to the #4 and #8 that were misfiring while the cat was plugged.

I also plan to remove the IAC, check it, and clean it. I will inspect the TPS and clean the PCV valve when I swap out the plugs. I have a box full of good sensors from chasing down those critters before realizing that the passenger side cylinder head was in fact the culprit.

Anyway, I'll be pulling and replacing parts on the truck early next week--I gotta work so I can pay for the parts I have already ordered! I will research a leakdown test for the heads, but I am curious about what standard vacuum values are for a Magnum at warm idle. And would any of the mentioned mods actually decrease the vacuum readings?

Thanks to you all!

(Pray for me: I have all the parts and hoses to replace my AC system but have yet to pull the dash. The writeup has given me enough courage to try...)

 

According to theory, should be around 20" vacuum at idle. Of course, a high mileage engine is going to be somewhat lower...... Size of the throttle body isn't going to make any difference at all at idle. Throttle bores are closed, and IAC is providing the air. If you have a stable idle, at the correct RPM, chances are good you are just fine.

 

True that on the closed throttle bore, I knew there was something logical I wasn't considering!

I try not to let the mileage (219,000) on the engine get to me as it had really good compression numbers when we did the heads. I did the main seals 2 weeks ago and could still see crosshatching on the cylinder walls, from below.

But I'll try to work sequentially on the tune up next week, rather than doing everything at once, and if I see improvement in the numbers I'll report back.

Thanks!

 

Is there anything to the old trick of rotating your distributor until you get the highest vacuum readings, or will the computer defeat that attempt? I've done it on some much older vehicles (******' without timing marks) in the past and called it close enough for ranch work...

 

Turning the distributor won't affect ignition timing, only injector timing. Really not a good idea.

If you can, do a cylinder leakdown test.... see what that tells ya.

Still the original injectors?

 

I replaced the injectors around the time I started hunting down a problem that eventually revealed itself as Cracked Cylinder Heads. For whatever reason, I didn't use NAPA injectors and I doubt that I used Bosch, either. I'm truly afraid I bought the set of replacement injectors off eBay...and of course the originals are no place to be found.

Depending on what testing tips I can wrest from this site I may pull a couple of low mileage injectors off a junker for testing purposes.

Thanks for the info on the timing; I'll take the FSM with me this weekend, and I'll report back with the leakdown test.

 

I still owe HeyYou a leakdown test, but when I started pulling spark plugs I noticed the #8 was dripping gas. Someone may not have made a good connection to the plug when rerouting the lines.

I replaced the Autolite platinums with AutoLite 3923 Copper (a colder plug), gapped them to .40 (they came out the box closer to .45-.50) and took it to town. At 18 mpg (65 mph) I'm not going to go look for trouble any time soon!

Thanks!

 

If it's runnin' good, and getting that kind of gas mileage, I don't think you have any more problems.

 

Holy mileage, Batman!

 

Lots of flat land down here, mileage drops to a steady 17 with cruise at 70mph, but for the mpgs I can (almost) drive 55!