Sporadic dip or surge in idle

[PhilisDiller]

Hey all,

 

Wondering if anyone might be able to help diagnose what’s going on here. When idling, the RPMs stay around 800 and it sounds alright but then every 15 seconds or so the RPMs drop down to less than 500 for a few seconds and it sounds like it’ll stall then it goes back to 800. Other times, at the same 15 second interval, the RPMs will jump up to around 1100 for a few seconds before dropping back down. 
 

My sig should have details on the MJ, but it looks like the last owner has put in the 4 port hole fuel injectors and had just replaced the IAC. 

 

 

she smells like she’s running a bit rich, but I’m not sure what these are supposed to smell like. Smells like a go-kart or lawn mower. Also, today I discovered that the cat had been hollowed out so that could contribute to some of the smell... 

 

I’ve just picked her up a couple weeks ago and am just diving in. I’ve attached the link to a couple videos. I haven’t determined any difference in scenarios between idle dip or idle surge. My first week I thought it surged more but this week it’s been dipping more. Still doing both.

idle dip

http://www.youtube.com/shorts/oC1tDuUP410

idle surge

http://www.youtube.com/shorts/Yraf39w-GM8

 

[jdog]

Have you performed cruisers tips?

Link in my signature

[Jesse J]

those tips solve everything I swear

[87MJTIM]

Sounds like the ECU is running in Open Loop (OL) at 800 RPMs.  When it switches to Closed Loop (CL) the RPMs drop.  If the ECU detect either ultra lean or rich condition, the ECU switches back to OL.  The RPMs will level out at 800, again.  After several seconds, the ECU will switch back to CL.  Just like shampoo instructions: Lather, Rinse, Repeat.

 

You will need to determine why it keeps dropping in/out of CL.  I would recommend you download the 88 wiring diagrams and the Renix fuel injection manual.

 

Also, do Cruiser's Tips for the Renix

[JMO413]

Cruisers tips are a must. Then start testing sensors.

https://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

This website is awesome for sensor testing.

[JMO413]

Cruisers tips are a must. Then start testing sensors.

https://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

This website is awesome for sensor testing.

[cruiser54]

Begin with Tips 1 through 5. Also make sure your intake manifold bolts haven't loosened up.

 

Make sure this tube has no vacuum leaks.

 

 

[87MJTIM]

If you haven't purchased one yet, get the REM from Nickintime.  Many Renix owners have one and love it.  I know, I am one of them.

[PhilisDiller]

Wow, thanks for all the great feedback guys.

Let's see where I'm at and what's been done so far

1 - grounds have been refreshed throughout. I'm going to be adding in the additional two grounding points recommended by Cruiser this weekend

2 - C101, I opened this up and cleaned this out a bit but decided I'm going to eliminate the C101 before I put too much effort into cleaning.

3 - I've refreshed the connectors for about 40% of the engine bay so far, starting on the drivers side including relays.

4 - Haven't done anything with Coil/ICM but will bang that out this weekend as well

5 - Haven't checked the sensor grounds yet but I plan on doing tip number 6 as well. 

 

2 hours ago, cruiser54 said:

Begin with Tips 1 through 5. Also make sure your intake manifold bolts haven't loosened up.

 

Make sure this tube has no vacuum leaks.

 

Thanks, but I've double checked all vacuum lines. the previous owner had replaced all vacuum lines in the last few years. 

Intake manifold bolts i'm doing tonight! I have a feeling this guy may be a culprit. 

 

 

14 hours ago, 87MJTIM said:

Sounds like the ECU is running in Open Loop (OL) at 800 RPMs.  When it switches to Closed Loop (CL) the RPMs drop.  If the ECU detect either ultra lean or rich condition, the ECU switches back to OL.  The RPMs will level out at 800, again.  After several seconds, the ECU will switch back to CL.  Just like shampoo instructions: Lather, Rinse, Repeat.

 

You will need to determine why it keeps dropping in/out of CL.  I would recommend you download the 88 wiring diagrams and the Renix fuel injection manual.

 

Also, do Cruiser's Tips for the Renix

This sounds spot on. I dove into the fuel injection manual and holy $#!& is it thorough. It does a great job with helping me understand all the components and their relation. I had the biggest smile on my face reading through it.

 

2 hours ago, 87MJTIM said:

If you haven't purchased one yet, get the REM from Nickintime.  Many Renix owners have one and love it.  I know, I am one of them.

I actually just discovered the REM last night and ordered one immediately! Seems like an invaluable little tool.

 

12 hours ago, JMO413 said:

Cruisers tips are a must. Then start testing sensors.

https://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

This website is awesome for sensor testing.

I've looked through this site before and it looks great! I went out an picked up a multimeter last week to help with diagnostics (moved back from Indonesia last year and have been repopulating my tools as I had to sell everything when we moved out there). Just been so busy with work and the new home that I haven't been able to jump in!

 

 

[87MJTIM]

Mine was running lean for a while.  I searched all components for possible causes.  My searches led me to an intake manifold leak.  I changed the gasket and it is no longer stuck in OL.  The ST and LT fuel trims have leveled off, also.

[cruiser54]

Most always simply snugging the bolts works fine.

[Tex06]

Keep us updated with your progress. Lucky you have a renix.....the REM is a fantastic little tool us H.O. guys can only dream about.
 

[PhilisDiller]

I ended up pulling the exhaust and intake manifold last night... I think I found the source of my $#!&ty O2 readings and running rich. I’m dropping it off at an exhaust shop to get it welded up today and will put everything back together. 
 

What’s the consensus on flex pipe to help take the stress off the manifold? Would I have the flex pipe installed on the Downpipe?
 

I managed to take a couple videos of the REM while driving and just like 87MJTIM said, as soon as it goes into closed loop the ST fuel trim starts just dropping to 0 until it errors out and goes back to open loop. This REM is such a @#$%ing amazing little device. NickInTimeDesign, you are a legend 🙇‍♂️  Best $ I’ve spent as it already saved me hours of headaches and wondering.
 

I’m going to do the motor mounts while I have the exhaust off. I was thinking about getting aftermarket mounts because I heard OEM ones fail quickly, but the previous owner already bought new OEM mounts and never installed them. I’ve heard some horror stories about vibration from some aftermarket motor mounts so I’m thinking of just reinforcing the gaps in the stock motor mounts with urethane window weld... but I know that’ll lead to more vibrations too. I’m not doing any off roading in her anytime soon and I want my lady to feel comfortable in the MJ with me.

 

Thanks guys!

I’ll update the thread once everything is back together. 
 

 

[cruiser54]

flex joint is a good idea.

[JMO413]

Flex pipe can't hurt. Motor mounts tend to be that subject everyone has an opinion on for sure. I have brown dog mounts on mine and am very happy. That's not to say I don't feel a little extra vibration. OE style will work just fine. Just keep an eye on them.

[PhilisDiller]

On 12/1/2020 at 5:43 PM, cruiser54 said:

flex joint is a good idea.

 

3 hours ago, JMO413 said:

Flex pipe can't hurt. Motor mounts tend to be that subject everyone has an opinion on for sure. I have brown dog mounts on mine and am very happy. That's not to say I don't feel a little extra vibration. OE style will work just fine. Just keep an eye on them.

I found an OEM NOS dead stock manifold for $220 and am thinking about pulling the trigger. I was thinking about welding the one I have up, but I’m considering it. I know the other OE style ones I’ve seen all have their fair share of issues so finding a factory one brand new seems like a win. 
 

think it’s worth the extra money? I’m being quoted $60 to weld

[Cheapamanche]

If you think that at any time down the road youbwill swap to the flex pipe manifold, I would just weld it. No sense in buying new just to replace it as soon as you get the chance. Save some money in the meantime.  Whats a couple extra days waiting to weld them.

[PhilisDiller]

26 minutes ago, Cheapamanche said:

If you think that at any time down the road youbwill swap to the flex pipe manifold, I would just weld it. No sense in buying new just to replace it as soon as you get the chance. Save some money in the meantime.  Whats a couple extra days waiting to weld them.

I don’t plan on buying the flex pipe manifold. I was planning on getting a flex pipe welded onto the down pipe, so I’d keep the manifold.  I’m just thinking of longevity, because I have heard of manifolds re-cracking. I’m just wondering if the flex pipe and the new motor mounts would be good enough to not crack it again. I’d gladly pay the $ to not have to weld it again in a year. 

[Cheapamanche]

I am a firm believer in doing what benefits me in the long run. Thats usually the smart choice.

[cruiser54]

weld it. Add a flex joint into the exhaust. 

[coolwind57]

10 hours ago, PhilisDiller said:

I don’t plan on buying the flex pipe manifold. I was planning on getting a flex pipe welded onto the down pipe, so I’d keep the manifold.  I’m just thinking of longevity, because I have heard of manifolds re-cracking. I’m just wondering if the flex pipe and the new motor mounts would be good enough to not crack it again. I’d gladly pay the $ to not have to weld it again in a year. 

I kinda went a step above and beyond, which I usually do in nearly anything.  I removed mine, repaired the cracks and then welded in gussets from the primaries to the flange.  I'm pretty sure I did at least the last 3 primaries.  I didn't want to deal with cracks ever again.  Removal of all that stuff was a J.O.B.

 

Reinstalled it and then had an exhaust shop weld in a flex pipe just after the vertical to horizontal bend on the head pipe.  Its been 3-4 years now and I've had no issues.

 

If you're welding those header cracks, I can't really see a disadvantage of adding gussets while you're at it.

[coolwind57]

Correction:  I used gussets on the last 2 primaries of the header.  Seems like that's where most of the stress was ending up.  Here's a shot of my flex pipe after I had it installed:

 

 

I'd also recommend that you have a look at your exhaust attachment points to ensure they're all present and secure.  Here's one at the bell housing:

 

 

 

Now here's one that I didn't even know I was suppose to have (and didn't).  This is at the transmission on top of the cross bar.  The exhaust shop showed this one to me and recommended that I actually utilize it.  Of course i agreed and they welded on an appropriate tab that you see in the pic..

 

 

Shockingly, that exhaust paint they used wore off in a day or two.  I got a kick out of them using it, deeming it pretty worthless but I guess it impresses some customers.  Got to hand it to them, it was a lovely thought.  

 

 

 

[PhilisDiller]

Alright, have done some work and definitely cleared up some issues but I’m still running super rich.

 

First I thought my O2 sensor was bad, so I replaced that with an NGK one.

 

cracked manifold was replaced with a new one - new gaskets

 

fuel pressure regulator was bust as my fuel pressure was reading 40 with vacuum and 47 without vacuum. Replaced my FPR and pressure is good 31/39  but still running rich.
 

here’s my synopsis

 

- running rich in open loop still. I know my O2 reads 0-5v no problem because when I decelerate at higher RPMs and the fuel is cut off, my O2 sensor reads right at 5. 
 

when idling though, I’m reading around 0.5v to 1.2v on the O2 (super rich)

 

all 6 of my spark plugs seem to have black soot on it so they were all running rich. I was thinking i maybe had an injector stuck open, but the identical spark plugs and high fuel pressure I figured they were all getting too much fuel via the high fuel pressure. The weird thing is that with the correct fuel pressure I can’t notice the difference in idle much. 
 

Although I’m glad to have replaced some faulty parts, I’m anxious to get her running good again. 
 
the fact it’s struggling in open loop makes me think there’s something funky going on.

the weird thing is that when it goes into closed loop and fuel is cut back to compensate for the rich ex, the O2 sensor reading actually goes from 1.2v down to 0.5v. I’d think that with less fuel, the O2 reading would be going up!

 

I'm going to go through and verify all my ground connections this weekend, but was hoping someone can help me. Here are my REM stats idling

O2 - 1.1V OL/ 0.5v CL

INJECTOR PULS WIDTH - 8.1mS OL/ 4.5mS CL

GALLONS PER HOUR - 1.1 OL / 0.8CL

MAP - 15.9 OL / 12.0 CL

VAC - 14.5 OL / didn’t grab closed loop

Water temp - 187 f

ST - 128 OL / drops to 0 in CL. 
 

 

I know it’s getting too much fuel, and i don’t think the ECU knows it’s sending too much fuel.

appreciate the help y’all and merry Christmas!

 

[jdog]

Is that water temp at operating temp?

[PhilisDiller]

18 minutes ago, jdog said:

Is that water temp at operating temp?

Yup. After idling for 10 min. My temp gauge on the dashboard is always quite low too.