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340 Monsoon - Idle hiccup issue (video) - May 1 update*


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I have a weird issue occurring at idle or very slow (<1000rpm) forward gear.  See the video here.  I trimmed the video, but the issue happens ~4 minutes apart in this example.

I'm not seeing any issues at speed - it runs really great and strong, other than this stutter at idle.  Also when you check out the video, you'll see the battery voltage drop then come back up, lagging the rpm drop by a second or two.

Recent maintenance items in the last 10-20 hours are plugs, wires, fuel filter, new distributor (was having a back fire issue late last year - I think the pickup coil ended up being the problem but swapped in a new complete distributor just to be sure and got timing set to 10 degrees BTDC). 

When I first had this issue happen, I thought the IAC could be the problem.  Put in a new IAC and TPS but the issue still occurs.

I also have a codemate scanner and am not getting any codes thrown.

Wanting to get a fuel pressure gauge on it this week so I can see if there's any PSI change when the stutter happens.  Anything else I should look for?

Thanks in advance!

Edited by dtm8119
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Look at any hoses or tubes involved with intake manifold.  Verify everything is tight like plenum and throttle body. Lucky for us, it's a simple engine and easily accessible.  Check your exhaust bolts too.

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The voltage drop lags the engine sputter so I don't think that's anything to sweat.  That's the juice from the alternator cutting out and back in I imagine.  Have you checked the IAC and TPS?  Those can go bad and are pretty easy to replace and/or clean.

 

EDIT:  Someone didn't read the entirety of the OP.  oops

 

Edited by UWSkier
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Clean the throttle body before you throw any more parts at it. 

Yours is a pain to do with the TB pointed down in the rear so the engine hood is in your way.  I have done it without removing the hood, but I had to use a flashlight to peek inside to see the parts while I was spraying them.  You really want the edge of the butterfly clean as well as the side of the TB throat.  Open the throttle with the engine off so you can direct the spray and get it clean.

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Thanks for the response everyone. 

IAC and TPS were just swapped in new, after this issue showed up.  When I put those in, I did clean up the throttle body with it in place - it seemed pretty clean.  I'll do another once over for hoses/bolts being hooked up and tight but I'm pretty sure things are good there.  

The strange part about this issue is that it'll idle rock solid at 650rpm for 3-4 minutes, do it's little hiccup/stutter, recover, and then continue to idle solid again until the hiccup happens again...over and over.  It also hasn't done it as the motor is warming up from a cold start - only seems to start occurring after it's gotten up to temp at 160.

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9 minutes ago, electricjohn said:

Could you have some water in your gas?

I couldn't 100% rule it out, however I think it's extremely unlikely.  Was stored nearly empty this winter, inside.  In season - stored on a covered lift.  15 gallons of fresh 91 non-oxy in it now. 

It runs very strong on plane and at ski speeds...no issues there.  Just this intermittent issue at idle.

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I assume your new distributor came with the module and pickup coil installed.  Pull the spark module at the base of the distributor and make sure it has enough thermal compound to carry the heat away.  You might also want to try swapping your old module back in to see if the problem changes.

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Did a hose check and decided to pull the PCV (hose wasn't on there that great).  Someone surprised as I assumed this was the PCV here but it's just a plastic 90 degree elbow with nothing inside.  Is that right...?

ICM checked and it had the needed thermal compound paste.

Still have to check fuel pressure.

KwFcMcP.jpg

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21 minutes ago, dtm8119 said:

Did a hose check and decided to pull the PCV (hose wasn't on there that great).  Someone surprised as I assumed this was the PCV here but it's just a plastic 90 degree elbow with nothing inside.  Is that right...?

ICM checked and it had the needed thermal compound paste.

Still have to check fuel pressure.

KwFcMcP.jpg

My 99 Monsoon II has the same non valve where you would think a PCV would be.  I have  a PCV on the other side.

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18 minutes ago, Bozboat said:

My 99 Monsoon II has the same non valve where you would think a PCV would be.  I have  a PCV on the other side.

Ok thanks, I see it now on the other side.

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OK here's a video of it happening with the fuel pressure gauge on.  1st hiccup at 30 seconds, the 2nd at 8 minutes.

Should I throw a new fuel pump at it?  Or fuel pressure regulator sticking? 

Thinking that I should also make sure it sees a constant voltage when the stumble occurs (if voltage drops...could be more of a computer issue, right?)

Got an appt with the dealer on May 12th otherwise to have them sort it out

 

Edited by dtm8119
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That's so fast, it's almost like an injector hung open for one cycle. 

Could be: injector hang, regulator, pump, blocked fuel line, blocked return line. 

My second vote is blocked return line.  It could have chunks inside, and the regulator eventually provides enough pressure to move them. 

You could leave the engine off and wire the pump on to see if it ever happens.  If not, it's an injector.  If so, it's fuel delivery. 

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OK so the voltage at the pump does drop when it stumbles.  I get a solid 14 volts running normally and then when it stumbled it went down to 12-13 volts, then back up to 14 volts as it recovered.

Should I swap in a new fuel pump relay you guys think and retry? 

Edited by dtm8119
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38 minutes ago, dtm8119 said:

OK so the voltage at the pump does drop when it stumbles.  I get a solid 14 volts running normally and then when it stumbled it went down to 12-13 volts, then back up to 14 volts as it recovered.

Should I swap in a new fuel pump relay you guys think and retry? 

Yes.  That's a cheap and easy part to replace.  Check all connections when you're in there too.

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8 hours ago, dtm8119 said:

OK so the voltage at the pump does drop when it stumbles.  I get a solid 14 volts running normally and then when it stumbled it went down to 12-13 volts, then back up to 14 volts as it recovered.

Should I swap in a new fuel pump relay you guys think and retry? 

That's normal alternator output.  It falls as the RPM falls.

If it's the relay dropping out you should be able to feel it click.  Better yet, pull the cover off and inspect it, lube the spring plate, and burnish the contacts.

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  • 2 weeks later...

Just to wrap this up (I hope!) - back from the dealer and first water test went well.  Ended up being the MAP sensor.  Weird that it didn't throw a code.

  • Like 2
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