Stumbling on Cold Starts
 

Gents,

I have an LQ4/4L80e that I am finishing up. Brendan @ lt1swap programmed the ecm for me. The first few times (first couple of weeks) I cranked and ran the engine, no issues. The last three times I've cranked the truck on a cold start, it stumbles heavily and didn't start first time two of those attempts. After the first time it did it, the next day, I cycled the key a couple of times to make sure the fuel was not bleeding pressure down. That did not help. After the truck starts and runs for a minute, all subsequent attempts to start are fine.

I have not road tested the setup yet. All attempts are in the garage.

Any thoughts?

Re: Stumbling on Cold Starts
 

Have you replaced the intake manifold gaskets? Stock ones start to crumble after this amount of time and create a big vacuum leak until the engine runs for a minute or two. Runs like it has a misfire and will throw codes for mass air flow sensor performance.

Re: Stumbling on Cold Starts
 

I did not replace intake gaskets, yet. It’s starting to need a rear main seal and I’m hoping to make it through this year without messing with the engine. I want to be 100% driveable before I hop it up next winter.

No codes yet. That also makes it weird for me. The only code I have ever gotten was when I forgot to plug the peddle in once.

Re: Stumbling on Cold Starts
 

Replace the knock sensors and harness while your in there

Re: Stumbling on Cold Starts
 

Were the attempts all in similar temps? (and is the motor mostly stock?)

Hot and Cold starts are affected by ASE or After Start Enrichment. On a somewhat modded motor, these values may need to be tweaked to prevent stumbling. If the temps drop and the air density spikes, the factory ASE on a modded motor doesn't keep up.

Re: Stumbling on Cold Starts
 

I think the temps were close. I live in Colorado and it fluctuates heavily, so who knows. As for mods, I’m bone stuck except no cats and full three inch exhaust. I drove it around for the first time today. Runs awesome and no codes.

Re: Stumbling on Cold Starts
 

I had a similar problem recently and found out it was a bad O2 sensor. Here is my thread.
http://67-72chevytrucks.com/vboard/s...ight=stumbling
Gets interesting around page 3.

Re: Stumbling on Cold Starts
 

Thank you for the heads-up. Tonight, I'm going to cold crank for the first time since test driving it a few miles yesterday.

Since I have a 100K plus motor, I'll start here and replace the O2 sensors with new ones. I'll follow-up tonight and after the O2 swap.

Re: Stumbling on Cold Starts
 

Quick update. Had my first cold start tonight. it had a very slight stumble before firing off. And, interestingly enough had a CEL. P0135 code, O2 sensor. So, I'm going to replace both O2 sensors like Wgesnerjr suggested and will report back in the next couple of days.

Re: Stumbling on Cold Starts
 

P0135 is a 02 heater circuit, could be heater element in the sensor or a ground issue. I cant remember what years silverados and other fullsize trucks/suvs it was but they had a common problem with a bad ground causing that code. The 02 sensor body grounded through the exhaust pipe itself, quick test was to command the heater on with a tech2 and if it didnt work take a piece of wire strip an inch or so bare and wrap it around the sensor body then put the other end on a good clean ground and try it again.

Engine doesnt run off of the o2 sensors until it hits a set temperature, cold starts are all off of the "base" tune aka open loop.

Easy check for intake gaskets leaking is grab a can of carb clean, brake clean, etc and spray down both sides of the intake where it meets the cylinder head BEFORE you start it the first time for a cold start. Spray it down real good then run around and fire it up real quick, if it idles higher than normal that confirms it. You can also smell the brake clean/carb clean in the exhaust if you put enough down.

Re: Stumbling on Cold Starts
 

That’s good, easy test. I’ll try that before I replace the sensors. I’ve already ordered the sensors. So, I’ll replace those regardless. Thanks.

Re: Stumbling on Cold Starts
 

I understand the ECU doesn't adjust Air Fuel Ratio until warm. However, in my case, the O2s DID have some kind of affect on start up. I think it's has to do with After Start Enrichment ASE, (thanks Br3w City) which is what old school guys would call a choke.

From the limited info I found, ASE is a table that is used to get the engine started and up to temp then the ECU would take over Air Fuel Ratio. Again, not much info out there. So I think ASE uses the heater in the O2 to heat up the sensor so the ECU can run the ASE table. My stumble was only for first 45 to 90 seconds or so. But the O2 heater did not work on my sensor and caused a rich mixture on that side of the engine. Here is the test I used for the heater check.

https://www.motor.com/magazine-summa...cuit-failures/

Also, in my case the sensor that caused my stumble was a Denso sensor installed during engine swap. But I think the exhaust shop I used may have welded the O2 bungs in with the sensor in place, there by damaging the heater part. I replaced with ACDelco and immediately cleared up problem.

Re: Stumbling on Cold Starts
 

I sprayed a ton of carb cleaner on the top sides of the manifold gaskets and then jump in and cranked it. No high rev or high idle. However, I should mention that it did not stumble either. I moved forward with replacing the O2 sensors and will post a message of the stumbling comes back.

Re: Stumbling on Cold Starts
 

I replaced the O2 sensors and will follow-up after a couple weeks. Thanks.

Re: Stumbling on Cold Starts
 

I had a similar issue a year after I had my truck running with no issues. Gave it some thought... stumble at startup = no fuel in lines. I checked my gas tank vent line and it dropped down from where I had originally placed it. Realized it was level or just below level and with the fuel sloshing around it had some fuel in the line that was causing a vacuum / clog. Cleared it out, moved the line back up and resolved the startup stutter, make sure your vent is clear.

Re: Stumbling on Cold Starts
 

unhook the maf sensor and see if it makes it run better

Re: Stumbling on Cold Starts
 

Sorry for bringing up such an old thread. But after 2 months, I have had no problems since replacing the O2 sensors. I know there are certain folks that swear the O2 sensors do not cause these types of issues. But, I am not having any more issues!