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Misfire? Engine Shudder? Vacuum leak? Where to start?


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I seem to be going backwards lately with my truck. I have had several things done recently that were more in the "should do" category than "need to do" category, yet each time it seems like the truck has taken one step forward and two steps back. 

 

I had the rear main seal replaced and it ended up leaking worse than before. Luckily I've been using this shop for a couple of years, they are very reputable and did it again for no charge, as expected. I picked it up last Thursday and dry pavement under the truck. Drove it back to the office and when I left work, dry pavement. Drove it about 20 miles that evening to my son's wrestling match, seemed to be running well, dry pavement underneath when I left the match a couple hours later. On the way home it started to shudder in the 0-40mph range under light acceleration. It was relatively fine while cruising at hwy speeds but would shudder again if I had to accelerate. My son drove it the past couple of days and I was able to drive it again tonight and it is definitely getting worse. Still dry pavement under the truck so the RMS replacement is holding up. I checked vacuum lines since one time they got crossed up at the airbox and it had similar symptoms but everything appears to be connected where it should be.

 

Looking for direction on what may be the cause of these symptoms. 

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My first thought would be to check that MAP sensor vacuum line. It runs from the MAP sensor on your firewall to a port on the side of your throttle body.  A poor connection or a crack in the hose effects critical vacuum, which makes your vehicle run like hell--especially at idle.  

 

Your symptoms are nearly exactly the same as mine when I discovered a bad connection here.   Mine put out a horrible unburned fuel smell at idle too.

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27 minutes ago, coolwind57 said:

My first thought would be to check that MAP sensor vacuum line. It runs from the MAP sensor on your firewall to a port on the side of your throttle body.  A poor connection or a crack in the hose effects critical vacuum, which makes your vehicle run like hell--especially at idle.  

 

Your symptoms are nearly exactly the same as mine when I discovered a bad connection here.   Mine put out a horrible unburned fuel smell at idle too.

 

Truck idles fine and seems to be ok, or at least is noticeably better, under hard acceleration. 

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I think it is the MAP sensor vacuum line. Drove it to work today and had a chance to fiddle with it and look around before running some errands a couple hours ago. Throttle body end of the brittle vacuum line seemed loose going in so I put a little pressure on it. Couldn't tell if it actually went in any further but maybe got a tighter fit, it definitely ran much smoother but not 100%. Will investigate further in a little while. 

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4 hours ago, JMO413 said:

Have you done Cruisers modification for that hose? It is cheap and then you know it's good!

 

Actually I did get the parts right after I got a bored TB from Mean Lemons, but of course I just kept putting off switching it out. I picked up a new Mopar TPS for cheap recently and was going to get a new MAP and IAC and just do everything all at once. Might have to brave the cold this weekend and just do it. Too bad my damn single car garage is just a storage facility. :doh:

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1 hour ago, JMO413 said:

I tapped my bored throttle body before I put it on. Mean lemons sure does good work!

Yes.  Same here.  Tap that bad boy when its removed from the engine. 

 

1 hour ago, WahooSteeler said:

 

Actually I did get the parts right after I got a bored TB I got from Mean Lemons, but of course I just kept putting off switching it out. I picked up a new Mopar TPS for cheap recently and was going to get a new MAP and IAC and just do everything all at once. Might have to brave the cold this weekend and just do it. Too bad my damn single car garage is just a storage facility. :doh:

I too have a bored out 60mm Renix TB from Meanlemons.  I'm very happy with it. 

That dumb plastic vacuum MAP hose (and the two pieces on the ends) causes more people to curse over irratic rpm and idle issues than about anything.   

 

 

 

 

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Just for reference....

 

THROTTLE BODY TO MAP SENSOR HOSE FIX

The Renix throttle bodies have a strange and failure prone connector on the side where the MAP supply originates and then runs up to the MAP sensor located on the firewall. This hose/pipe assembly is no longer available for purchase.

The real kicker here is how critical this line is in supplying the correct vacuum signal to the MAP, the most relied upon sensor for the ECU to read regarding air/fuel ratio. Any cracks, melted spots, or loose rubber connectors can cause major starting and driveability issues.

There’s a simple fix though. All that’s required is a 1/8” NPT tap, a new throttle body gasket ( Napa FPG 60742 ), a vacuum fitting (Napa 05703-B102), two vacuum elbows (Napa2-670), and a length of new plastic piping (Napa 2-672).

1-20160126_155433

Remove the throttle body and take it to the workbench.

Using an oiled tap along with a driver, carefully thread the lower of the 2 holes of the throttle body where the old fitting was plugged in. Don’t go too deep. These are pipe threads.

Flush the hole with carb cleaner and inspect for any left over cuttings.
This is an excellent time to do a complete throttle body and IAC cleaning.  See Tip 11.
Take the vacuum fitting (05703-B102 ) and apply a LITTLE bit of thread sealer on the threads only. I prefer Permatex #2 but almost anything is fine.

.1-TB to MAP hose-001

Carefully screw the fitting in until snug.

Install one of the vacuum elbows on the MAP sensor so it points toward the throttle body, and the other vacuum elbow on your new throttle body fitting so it points up to the MAP sensor.
Cut a length of the new plastic tubing (approximately 13 inches) to fit between the vacuum elbows and install it making sure there is enough slack for some engine movement. Route it according to the photo. We don’t want any rubbing or chafing with engine movement.  Not a bad idea to use some contact cement or Gasga-Cinch sparingly on the tubing to elbow connectors.

1-TB to MAP hose

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Cruiser, thanks for the re-post. I have your tips saved and as stated in a post above have already bought these parts to install with my bored TB. Question though, I bought the plastic vacuum piping from your parts list, but there is rubber tubing available with the same orifice size that obviously would not become brittle and break as easily as the plastic tubing. Any reason that would not work also? 

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