Vacuum Pump or slight misfire? - Dodge Cummins Diesel Forum
07.5 - 09 3rd Gen 6.7L Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine, Transmission, etc...NO ADVERTISING

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post #1 of 7 (permalink) Old 03-31-2019, 05:35 AM Thread Starter
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Question Vacuum Pump or slight misfire?

Hello, all!
Quick run through on what I'm experiencing... Hoping for some advice on this:

First:
I've always had some lag when switching HVAC controls from dash vents to defrost and what-not, i know this is normally a vacuum pump issue or blendoor. Never thought twice about, never really cared. Window is normally down in the summer anyways and it basically stays on Defrost in the winter. No problems there.

Second:
The truck has 240K miles on it now and still has factory original injectors. (Verified with last 2 owners )

Third:
Got into a pretty serious accident a some time ago and got it repainted and the whole shibang. When I got it back, come to find out, when they painted all the new fenders, fender supports, and coated the frame, they attached all the grounds to the new painted surfaces which didn't help my already corroded, old, weak battery cables and grounds. This made the truck have a hard time stay running, starting, and when it would run, it would run really rough and through all kinds of codes. A few weeks of diagnosing and help from you guys, I replaced all of the battery cables and grounds from Custombatterycables.com and it seems to have solved all my problems.

Now on to the problem - After I replaced all the cables and paid more attention to my RPM making sure it wasn't surging like it was when I was having electrical issues I noticed that it had this really really slight surge at idle both hot and cold idle. Voltage stays at 14V the whole time (unless the grid heater is cycling). It's just enough for me to notice even outside the truck.

According to my Edge Insight - It drops ~50 - 100 RPM every minute or so. Then, just to throw me off, it will spike ~50 - 100RPM. Which leads me to believe it's a slight misfire caused by a failing injector. They're old... So I wouldn't be surprised.

Here's the kicker: It only does it when I have the blower motor on. Doesn't matter where it's blowing, hot or cold, A/C on or off. But if I turn the fan completely off. It doesn't do it at all.

The truck runs great though. To be completely honest, it could have been this way for as long as I've owned it and only noticed it because I was paying super close attention to it, looking for any kind of symptom after replacing the battery cables and grounds.
No Blow-by, no voltage drops, no smoke. The only other thing I can possibly think of that would lead me to injectors is that my fuel mileage has dropped by about 1MPG in the last few months. Since I'm in Utah and we switched to winter diesel before I got the truck back and most fuel stations still haven't switched back to "Summer Diesel", I feel that will account for the decrease.

Thoughts? Suggestions?

I am known to over think things and be a worry-wart when it comes to this truck so there's that...


2007.5 DODGE RAM 3500 QCDRW - 6.7L CUMMINS FULL DELETE - H&S INTAKE HORN - AFE AIR INTAKE - ARP HEAD STUDS - EFI LIVE BY STARLITE DIESEL - FASS TITANIUM - BEANS DIESEL SUMP - STRAIGHT PIPED - G56 6 SPEED TRANS

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post #2 of 7 (permalink) Old 04-01-2019, 02:28 AM
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Your truck doesn't have a vacuum pump so you can rule that out. It could be the A/C compressor cycling on since you only notice it when the blower motor is on. Even if you don't have the A/C on, the compressor will still cycle when you are on any of the defrost setting.
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post #3 of 7 (permalink) Old 04-04-2019, 07:26 AM Thread Starter
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Quote:
Originally Posted by KhakiCummins View Post
Your truck doesn't have a vacuum pump so you can rule that out. It could be the A/C compressor cycling on since you only notice it when the blower motor is on. Even if you don't have the A/C on, the compressor will still cycle when you are on any of the defrost setting.
If there's no vacuum pump; does it have a hydroboost off the power steering? There is a hissing noise when turning or applying the brakes really hard.

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post #4 of 7 (permalink) Old 04-04-2019, 01:31 PM
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Yes, your truck has a hydroboost brake booster.

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post #5 of 7 (permalink) Old 04-08-2019, 09:24 PM
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The HVAC vents are all electric motors, they get slow when they age and the vents are really crappy so they could be sticky and jamming and the controller could be getting bad. Unfortunately the only good fix in new doors, motors, and controller.

Change your fan motor and see if that makes a difference, it could be drawing too much and throwing garbage into the electrical system. Also check for any missed grounds on the firewall or under the dash and any accessories wired directly to the battery, that is a BIG no-no.

Could also be injectors lacking extra filtration, additives, and care selecting fuel. If they are suspect change them NOW, you do not want to the results if a tip comes off an injector are they are over fueling too much. Think windowed block, melted pistons, and general mayhem.

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post #6 of 7 (permalink) Old 06-15-2019, 08:52 AM Thread Starter
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Quote:
Originally Posted by cerberusiam View Post
The HVAC vents are all electric motors, they get slow when they age and the vents are really crappy so they could be sticky and jamming and the controller could be getting bad. Unfortunately the only good fix in new doors, motors, and controller.

Change your fan motor and see if that makes a difference, it could be drawing too much and throwing garbage into the electrical system. Also check for any missed grounds on the firewall or under the dash and any accessories wired directly to the battery, that is a BIG no-no.

Could also be injectors lacking extra filtration, additives, and care selecting fuel. If they are suspect change them NOW, you do not want to the results if a tip comes off an injector are they are over fueling too much. Think windowed block, melted pistons, and general mayhem.
Sorry for the late reply.

As for your suggestions, I haven't looked into the electric motors. I'm guessing this might be the case though... It often takes time to switch vents and the A/C Didn't blow cold unless I was just cruising under 50MPH. So... I'll get that fixed after I fix the A/C... It exploded recently.

As for the grounds, Late last year after getting into an accident and having the truck repainted, I had a million and half electrical gremlins to deal with and re-did the entire electrical system; New batteries, Battery cables, starter cable, Grid heater cables, alternator cables, ALL new grounds (engine bay only), and cleaned down to bare metal for all the grounds and sealed them. So I don't think that's the issue.

As for injectors, they were my first inclination. I'm at 250K on original injectors. How-ever, I feel as-though they are not suspect since it's only occurring when the HVAC system is running and is at idle. I do have slight injector tick at start up, but it goes away past 120F and I run Power Service Injector Cleaner/Lube every other tank, as-well-as change the fuel filter and water separator on my FASS Pump every 10K Miles.

I did however wire my flood lights directly to the Driver's side battery because I didn't want to tap key on power out of fear of the TIPM. I've had 1 too many nightmares with the TIPM and no desire to mess with it ever again. I was experiencing this issue prior to installing them though.

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post #7 of 7 (permalink) Old 06-29-2019, 09:08 AM Thread Starter
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Here is the picture of the exploded AC.

Now that ambient temperatures have warmed up, It hasn't been "surging" at all. Event with the A/C or Heat on.
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2007.5 DODGE RAM 3500 QCDRW - 6.7L CUMMINS FULL DELETE - H&S INTAKE HORN - AFE AIR INTAKE - ARP HEAD STUDS - EFI LIVE BY STARLITE DIESEL - FASS TITANIUM - BEANS DIESEL SUMP - STRAIGHT PIPED - G56 6 SPEED TRANS

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