New engine, now running rich and rough idle …

[Carlos]

I just had a pro rebuilt engine installed in my 2000 Jeep Wrangler 4.0 automatic. It now shakes and wants to die at idle speed but is 100% perfect at all other RPM ranges and has good power overall.

When coasting down to stop at a stop light it will shake pretty bad and possibly die, it has died 3 times at stops this week.

I am no expert in diagnosis but i`ve spent the past few weeks watching videos and testing everything I can with my laptop scan setup and also my cellphone with OBDWiz or whatever its called. I also have the factory service manual.

Fuel trims at idle are typically around negative 18% – anywhere above idle, even 900 RPM and things go completely normal. Before the new engine it was consistently at 4-6% positive fuel trim. The engine was replaced due to a broken piston skirt.

So my big concern is whether or not this is a coincidence or something worse going on inside the engine. Here is what i`ve tested or swapped from my good running 2000 XJ parts:

• Fuel pressure: 45 PSI, completely within spec according to all procedures in the FSM.
• Cam timing: My scanner just says “in-sync” but i`m waiting on a DRB III to get here so I can fully verify – although I did flip it 180 out and even unplugged it with zero change.
• MAP: Numbers look fine, plugged in a spare known good one with no change
• Coil Rail: Swapped to known good with no change
• Injector balance test: The old Siemens injectors were out of range by about 3 PSI across the group – swapped to rebuilt Bosch 4 holes with no change
• IAC: Was a little dirty, so put in a known good one that is nice and clean, no change
• Vacuum: Unplugged and capped off everything with no change
• o2 Sensors: Discovered one wire that had a bare spot & one with a cracked plug – changed both upstream with new OEM, no change
• Discovered 180 thermostat from shop who did install, put in new 195 and a new temp sensor, no change (now runs at 210~)
• Adjusted throttle valve cable for auto trans
• Replaced new champion plugs with correct Autolite APP985

Things I have not yet done:

Compression test
Verify good cats – the engine runs so well with such good power that I don`t really suspect these
Fully verify cam/crank sync – though I doubt this is an issue, since i`ve messed with these quite a bit and never had these symptoms due to that sensor – i`ve also carefully aligned it all with the toothpick and TDC method, including 180 out. This will be verified soon with a DRB 3

Observations:

SInce this is happening on both banks, I believe it must be something external .. like MAP, IAC, IAT, who knows?
On first start up before entering closed loop, there are no symptoms at all
In my mind this is a simple issue of the computer pulling fuel for whatever reason, and thus causing the rough idle and possible stall

I am 100% open to testing anything anyone suggests, and following through with the final solution here. A year ago I went through the same thing, except for a random misfire which ended up being a non Mopar crank position sensor, that was a huge relief to discover.

I have driven it pretty hard for a few hundred miles since the new engine – just to break it in and so on.

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[Steve Klein]

I am assuming that there are no codes (pending or hard)? If you are running good prior to closed-loop, you need to be looking at the 02 sensors because they are the main input for closed-loop. I would also verify the TPS is working properly (good idle voltage and WOT voltage).

[Carlos]

No codes at all. I have a decent scan tool and have watched the IAC steps and TPS move freely. I also tested the TPS and MAP on the bench with a power supply, both do seem to be moving smoothly through their range, and within range. I agree though: TPS, IAC, MAP, and o2 are the only things that make sense.

o2 were replaced yesterday with brand new OEM NTK units, with no change.

Also, i`ve reset the pcm each time and let it relearn everything. Within seconds of going into closed loop the numbers start counting -2, -5, -8, -12, -15, -19 STFT then severla minutes later those become LTFT.

[Steve Klein]

Have you verified the vacuum line to the MAP is good (preferably new). No cracks, etc?

[Carlos]

Yes, and i`m currently putting the TB back togehter for another test .. just pulled and verified all the sensors and gaskets, o-ring, etc are good.

[Steve Klein]

How much vacuum does the engine read at idle (with a gauge)?

[Carlos]

Looks like about 10 in Hg

Also attaching a photo of the fuel trim after putting the TB back on, both cold and warmed up. Idle starts loping once its at operating temp.

Vacuum:

Operating Temp:

Cold:

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[Steve Klein]

You should have better vacuum than that…closer to 20″. I think that might be your issue (MAP seeing ‘engine load’ and adding too much fuel). Now, why is the vacuum so low? Mechanical timing needs to be verified. Need to check for intake vacuum leak. If that is good, you need a leak down test.

[Carlos]

Sounds good, I won`t be able to verify internal timing as far as the chain goes or a leak down test here. I can do a compression test. Does this point at all to clogged or melted down catalytic converters?

Timing of the DIS/fuel injection timing would not affect vacuum right?

[Steve Klein]

I don’t think a clogged cat would affect that at idle. Since, as you mentioned, you have good power higher in the RPM range, that is unlikely anyway. Fuel injection would not affect it at all. But do check for a vacuum leak (use carb spray, carefully) around the intake manifold.

[Carlos]

Thanks for everything, will try that and also talk to the shop about checking timing and so forth.

[Billy]

Agree compression test is next step. 10″ vacuum is super low and suggests mechanical timing or other mechanical problem.

[Carlos]

Compression test coming next this evening. Was not able to find any intake leaks with starting fluid. I capped off all intake ports again while using the vacuum gauge and it went up to about 15 hg, but fuel trims also climbed up over 30 at idle, but promptly dropped when enriched with small amounts of starting fluid, lol.

Timing cover does look easy to get to, but i`m afraid to get into there and risk any warranty there might be – waiting to hear back from them on this. I can`t really imagine an engine coming from a rebuilder without properly aligned timing chain/sprockets. We will find out.

[Billy]

[quote=”CarlosA” post=195200]I capped off all intake ports again while using the vacuum gauge and it went up to about 15 hg….[/quote]

Wait, are you saying you had intake ports open during your prior vacuum test? To be clear, there should never be any ports open to atmosphere downstream of the air filter. I haven’t gone back over the whole thread to see if you’ve done this yet, but a smoke test should be one of your first steps.

[Carlos]

No, just capped off from wherever they go .. climate control, brake booster, evac, etc. I did that to rule out any leaks in those places.

[Billy]

Sounds like you may have some leaks in one or more of those places as well. Once you have the main problem fixed and a nice 20-21″ vacuum, I’d reconnect each of those hoses one by one and see what loses you some vacuum. First things first, smoke and compression.

[Author]

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