Hey folks,
I'm going through break-in process of a rebuilt engine, haven't done much more than about 20 miles on it, so I understand that there's probably some amount of adaptations that the DME still needs to do. But was curious what y'all would say about my current behavior.

My car will crank up from cold nice and quick, no hesitations, no oddness of any kind. After about 25 seconds, a rough idle starts and it appears that the car starts leaning out a bit too much. So much, that if I rev it up a bit (say to 2K or 3K) and let go of gas while on neutral, the car will just past idle and stall.

Eventually after letting it get to hot, the rough idle gets better (and is less lean) but the stalling still occurs. Not when dropping from 2 or 3K (it manages to recover to idle through a bit of a studder), but if i'm at 4K or so, it definitely stalls.

No codes of any kind; passed a smoke test through intake/boost. Pending is an evap leak (i did have an issue with the fuel pump housing not being seated right. Resolved it, but rough idle/stall problem persists). Injectors have been flow tested and cleaned. Different cables, plugs and coil. New MAP and TMAP sensors, CPS. New PCV and evap solenoid. Fresh Techron 91 in the gas tank. Don't know what else to check.

There's a post out there from 10+ years ago (https://www.northamericanmotoring.co...ml#post3333227) about someone running into similar issue and resolved it by having engine suck on some Seafoam. Read so much about Seafoam not really intended for 4-cycle engines, so I'm afraid of just doing something like that to my new engine. Curious what y'all have to say.

Thanks for any feedback.

 

My last engine was doing something like this, but with more problems. I replaced the engine after the AFR was always all over the place despite trying to tune it through bytetronic. I found that when I had your symptoms my car would run too rich try to recover by leaning out and that was the cause of it dying. Mine ran like a bad lawnmower when cold, but once it was warm I could turn it off and back on it would be a completely different car until it cooled down. Pretty sure I have bad rings in that engine too though.

 

Sounds like an issue when you get into closed-loop. O2 sensor, perhaps?

 

O2 sensors (both up and downstream) are new.

It is indeed occuring when closed loop is active, so that sent me on a path to swap/replace sensors. Only sensor I haven't swapped/replaced is the crankshaft position sensor and the coolant/temp sensor. I figured if CPS is bad, I would be having way more troubles than just a rough idle... and I'm able to see the temp over scanner and falls/rises correctly from what I can tell.

Any other thoughts? 🙂

 

*bump*

Just got back from vacation and really would like to tackle this. Any other thoughts?

Some threads online are suggesting an issue with fuel delivery still. Might swap my new fuel pump (which is not OEM, it's Hamburg Tech, got it from ECS Tuning), with my previous one (OEM) and see if there's any difference. Though still not sure why a fuel pump problem would get better when car is hotter than when cold.

Any thoughts are appreciated.

 

If the engine coolant temperature sensor is bad providing incorrect temperature readings, in this case likely higher than actual, the engine controller will not fuel the engine correctly because it has the wrong -- too high -- temperature. At cold start and for a some time after the engine controller does not rely upon the O2 sensor as it is not up to operating temperature. But once the engine is warm and its temperature more closely agrees with what the coolant temperature sensor reading is then the engine will run better.

I had a car -- 2002 VW Golf TDi -- that manifested cold start/cold idle misbehavior. Yet ran just fine when warm. Suspected the coolant temperature sensor. I had an OBD2 scan tool and used this to observe the sensor temperature being supplied to the engine controller. I don't recall the exact numbers any more but the temperature reading didn't agree with what at cold start should have been ambient. Had the car in and requested the coolant temperature sensor be replaced. Afterwards the engine ran fine.

 

Thanks! Coolant temp sensor is one key sensor I haven't yet replaced, I'll take a look at it!

I did look at the live data and it didn't seem off. The car gets hot and fans kick in and all that good stuff. But Im coming to learn that there is responsiveness from sensors that the DME expects and a failing sensor may not be responsive enough and cause problems. Will look into it.

Any other thoughts?

 

All,
I have since replaced the coolant temp sensor and the behavior is the same. I've also verified through scanner ... coolant temp readings make sense.

At this point I'm out of ideas. I'll be soon taking the car to a mechanic friend and toss him a few hundred bucks to spend some days with it and diagnose away. He and I hope there's still some vacuum leak somewhere that's causing the engine to run lean when cold. The injectors then are flooding the engine with fuel. The leak probably gets less bad with a hot engine (?).

At this point it's only a nuisance. Car will stall when I approach my nearest two or three intersections. If I try to turn it right away, it sounds as if the starter motor didn't have enough juice (very weak). I believe this is most likely that there's lots of fuel in the compression chambers. If I let the car sit for like 5-8 seconds and then turn it, the car starts as usual with normal sound of the starter motor (I assume because fuel seeps away?).

Once the car seems to be warm enough (or whatever), the issue sort of goes away. I say sort of, because I still see that the needle dips under 750 RPM slightly, car shudders for like 1/4 of a second, and then idles perfectly at around 800 RPM. As if the CPU wasn't ready to do idle speed adjustment and allows RPMs to drop before it does it's idle speed management things ...

Also, during this cold start, the car runs like it's pulling a big *** rock in the back. I have to press the gas more than usual to get it to go. I'm sure this is again because the car is running way rich and combustion is very weak.

Any further thoughts/suggestions are appreciated.

 

check the accelerator pedal throttle position synch with a volt meter. Also take the intake boot off and see if the plate is fluttering when idling.

 

Thanks, I'll check, though I'd be curious whyy any of these would only be an issue only at cold / first start in the morning. I'm however open to look at anything at this point, would love to get my ride to 100%

 

Cemocn, how's it going with the car? Nothing more frustrating than spending all that money on the engine/trans only to continue to have a drivability problem. Using your scan tool in the OBDII mode (not under the MINI menu), pull up your STFT and LTFT. what are your percentages at idle? Repeat at 3000 RPM. Post your numbers.

When you have an engine that's hunting for the proper idle speed, all sorts of sensors can cause this type of problem from MAF, coolant temp (especially on a cold start), MAP, O2 etc. Mechanical defects (like valve train) issues can also cause this problem. Fuel trims can give you a direction that you need to go when troubleshooting, but first, you need to understand how fuel trims work, and how the engine computer will react to the various inputs it receives. Sometimes, I've see sensors lie to the computer and that cause the computer to react incorrectly.

Watch this video.