Rough Idle on cold start 5.3L

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
Recently my 5.3L has been giving a rough idle on cold start only, no drivability issues. It starts fine, goes to 900/1000RPM as usual and after a few seconds it starts to stumble. The RPMs drop to 500 and bounces to 700 for a few seconds, 5-10 maybe. Then it slowly starts to clear out. the idle will jump to 1100 then settles down at 900 and then slowly drops as per normal warm up. Whole process from start, rough and back to smooth is under 20 seconds.

Current temps are single digits in the morning, approaching 10C. Doesn't do this except first thing in the morning. 145k km (90k miles). Recent new plugs, wires, O2 sensor (done by dealer). Also a new air filter. No CEL present.

I found other threads with similar symptoms. Those threads all seemed to have a lot of guesses at the problems and no resolutions. One thread mentioned "SAIS Valve" but I don't know what this means? I am guessing it is an air injection system? Anyone point me in the right direction on that?

Has anyone had this issue and solved it? TIA
 
Last edited:

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Do you notice a heavy draw on the voltage gauge? As I understand it, the Secondary Air Injection System only operates at a cold start and the air pump is a big power draw.
 

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
A very good question! I don't know yet. I was focused on the tach and the oil pressure before so I will watch for that.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
No SAIS on the V8.

I'm thinking the MAF or MAP sensor. Maybe try cleaning the MAF and reset the PCM.
 
  • Like
Reactions: Tiggerr

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,642
Tampa Bay Area
This is a painless suggestion, but it might actually surprise you with good results... Pull the Brake Booster Line and clean the Grommet-to-Hose interfaces on both ends and clean both of the plugs-in points. Then after inserting them back in place good and snug, try the start up when the engine is ice cold and see if this helps stabilize the idle. Was the Throttle Body cleaned 'on the bench' yet? That might un-stick a sketchy Butterfly Plate and help as well. ;>)
 
Last edited:

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
Thanks for the other tips.

I thought the throttle body cleaning issue was more of a I6 issue than a V8? I also thought that a vacuum leak or dirty throttle body would have given symptoms while driving. This issue is only during a cold start and otherwise drives great.

If I have time this weekend coming I will try some of those things. In the mean time we are in a heat wave with daytime highs in 30s and warm mornings. The stumble is still there though less noticeable.

Oh, and there is no voltage pull on the gauges showing to a previous comment.


Looking forward to hearing the experience of someone who has had this exact problem and solved it.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
It typically is a 4.2 issue with the dirty TB but they can get a little gummed up on the 5.3, possibly restricting idle passages. It can't hurt to clean it.
 

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
Agreed, can't hurt. I kinda have a very finite amount of free time and was hoping to find the smoking gun on this before starting. Maybe it's one those issues where you have to try 10 things... if so I might take it to the dealer. ☹️
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
It's not that bad on these. Very easy to get at. I doubt even the dealer would have any luck except throw parts at it until it's fixed.
 

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
So my update is: no update. The weather got warmer, the problem has gone. It's been as low as 13°C and there hasn't been any notable rough idle. Hard to troubleshoot an issue that isn't there. I will try to report back if anything new develops but I dunno when that might be. Maybe it was a bad tank of gas that the start up routine didn't like...
 

AWD V8

Member
Jan 12, 2015
463
The MAF is easy to eliminate. Unplug it and drive it a few miles. The PCM will learn to rely on the other sensors and ignore the unplugged MAF. If the MAF is causing the problem it will be gone. If the problem is still there it's not likely the MAF. So, unplug it and drive the day before and in the morning you will know. You will have a CEL for the unplugged MAF. When you first start it after unplugging the MAF it will likely stall or stumble for a few seconds until the PCM realizes it can't use the MAF.

Do you have a scanner to read data? I'd be looking at the ECT before you start it. Compare the reading to the actual temp. If the coolant sensor is giving a bad reading the mixture could be too lean or too rich, causing the problem.

The O2 sensor will take a minute or so before the PCM looks to them for readings.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Just one warning unplugging the MAF on the 5.3. Shifts may be brutal, especially 1-2 shift. When I forgot to plug mine, felt like I had a shift kit set to the hardest firmness.
 

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
If the MAF was an issue would I not see drivability issues? It has no driving issues at all atm.

The engine coolant temperature switch was replaced a couple years ago (needlessly, but that's a different story) so I'd be surprised if that was an issue. Temp gauge seems to work well.

I have a typical OBD2 scanner for CEL codes.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
If the MAF was an issue would I not see drivability issues? It has no driving issues at all atm.

Not necessarily. It could be gunked up and not giving accurate readings, slightly varying air/fuel ratio. As an example, I had a loose intake hose before the throttle body but after the MAF that was partially disconnected and letting in air. So this was unmetered air past the MAF. I never got a code and it drove fine.
 

Bow_Tied

Original poster
Member
Dec 21, 2014
453
London, ON
I'll look into it, thx. Hope it'
s that easy. :smile:
 

Forum Statistics

Threads
23,273
Posts
637,499
Members
18,472
Latest member
MissCrutcher

Members Online