Stumbling after valve cover job

My '03 DC pre runner had a super slow valve cover leak which was dripping on the headers making me feel embarrassed, lol. To address it, I replaced the valve cover gasket, re-sealed the half moons, new cam seals, and while I was there, new spark plugs and wires. I also reset the ECU, and changed the oil and fill filter. The leak has stopped! Great news!

Unfortunately, I picked up a running issue (that was not present prior to the work) since completing the job: a few "stumbles" when the truck is cold. Sometimes I notice it as it idles - just a couple of what feel like misfires. It's most noticable if I am climbing a hill at lower engine speed, like 1900-2200rpm or so, I will get 3 to 6 "stumbles". Otherwise the truck runs solid, and after running say 5-10 minutes, I haven't noticed any stumbles. Also, no CEL activation as of yet, and I've put ~100 miles on it since I got it buttoned up. Stumbles happen EVERY time I start it from cold so far, but hasn't happened while the truck is warm, or after a warm start.

My intuition is that there is a possible vacuum leak - one of the vacuum hoses that connects to the air box from the steel tube above the plenum was split when I got to it, and I cut it back to expose some non-split hose, but it did split just a bit but appears to have made a good deal. I taped it with electrical tape and zip tied it, and have silicone replacement vacuum tubes on the way.

Just looking for any other advice or thoughts that you all might have to help me get this thing back to ship shape.

 

Did you check for codes? Pending codes can be stored in the ECU without the CEL on.

New spark plugs and wires- Did you replace the plugs with the correct twin electrode plugs? What brand for the wires? Have you tried removing the coils and wires and reseating them? Did you clean the coils? I would also check any harness that you may have touched while doing the valve cover gasket job to ensure they are completely seated.

 

Make sure you tighten down your ground wires.

 

Thanks for the replies!

•Ground wires tight: Check
•New plugs correct dual electrode style: Check
•Plug wires: NGK (same as what was on there before which worked fine)
•Checking for codes: I don't have a reader (lent to a friend who is MIA ever since I asked for it back like a month ago!). I will either buy another one or go get a free read at a parts store if I can't get the symptoms to stay away
•Harness conections: re-detached and confirmed with a "click" just now.
•I actually JUST pulled the driver side boots and reseated, and pulled the plugs off the outside of the coils on the passenger side and reseated - and no stumble on my drive home! Will have to drive again later this evening or tomorrow to confirm, but this is the first time I haven't gotten a stumble while cold, so it feels like something changed. Fingers crossed

 

Welp, unfortunately, I've tried all the suggestions, and I got a stumble today... This time, well after the truck had warmed up. I am hoping to get my obdii reader back tomorrow, so I'll check for codes at that time.

 

My truck had the same problems after the valve cover gaskets were resealed. I didn't do the work myself, a shop did. My truck was in the 220/230k mile range when I had it done. I was also about 50k into a set of spark plug wires and coils. The first problem was that the stock airbox was cracked during reinstallation and there were vacuum hoses not reinstalled. I found the hoses myself first which helped a lot then saw the airbox problem. I replaced it as well. I understand that old plastic breaks, but the shop tried the stereotypical bullsh*t to pass it off as it was previously broken(wasn't)/oh didn't notice/they break when old routine. Tell me it broke and I'll appreciate the honestly and pay for the part...Last time I took the truck there. Anyway, curing those vacuum leaks cured 95% of the stumbles. After a week of the occasional misfire mostly under heavier load, I replaced the wires & coils. Another mechanic friend told me that the coil/wire system can be a bit finicky about being removed/reinstalled when they are older. Don't know if it's true but it did the trick. No problems and the truck is now at 290k.

 

^thanks for this. I'll get my hoses in later this week and install them next week when I'm home from a trip. I'll do the hoses see where that gets me first, then I guess coils if need be

 

Check the very back upper side plenum 1/4” vac line (between firewall and plenum), there is one that gets overlooked that is only 2-3” long and can get folded under when reinstalling. May go unnoticed and
It may be the issue

 

^i did confirm this morning, that tube is as it should be. Thanks for the suggestion!

 

Vacuum leak for sure.

Get it smoke tested.

 

^good call, best option for smoke creation is some Palo Santo. If it doesn't detect vacuum leaks, at least it'll cleanse the engine of malevolent spirits! Next best thing is I have a neighbor who vapes, lol. But yes, absolutely I will get some smoke in there before I put in the new vacuum lines

 

If it didn’t have this symptom before, and does now after the work you did, I’d focus almost exclusively on things you touched.

If the obvious vacuum, spark and electrical connections check out, maybe double check both plenum and throttle body gaskets? If you reused one in poor condition or installed with the incorrect orientation, that could cause a vacuum leak.

A quality OBD scanner that reads live fuel trim data may help you diagnose.

 

Many of you suggested, like I had originally assumed, that there was a vacuum leak. My Palo Santo smoke test confirmed right before I put on new silicone vacuum lines on every easily accessable line (I didn't remove intake to get the three lines under the intake tube). Now 300 miles of ECU reset miles, and no more stumbles! I also managed to get the sensors ready just in the nic of time for my registration smog check, which it passed!

Thanks for holding my hand y'all! Next on the maintenance list is new tundra brake upgrade next week, looking forward to getting rid of the shuttering front discs!

 

Way to go! These vacuum leaks are always a huge PITA to track down. Always extra satisfying when you can fix it yourself!

 

Indeed! Nothing worse than fixing one problem but having another pop up - so this is extra satisfying.

This is my second Tacoma, the first being a 96 five lugger, which apart from a clutch job early on at a friend's shop, had only ever saw me as it's mechanic for the 8 years, 80k miles. Lots of space in the engine bay made work on that vehicle a breeze. When I bought this truck, I knew that there would be some deferred maintenance to catch up on, so it's nice to be checking boxes!

 

Glad you got it fixed. As a side note, because of the age of these trucks, the wires that lead down to the Crank Position Sensor that have withstood years of vibratory abrasion usually get disrupted during VC gasket and timing belt replacement jobs. The wires are bare but still insulated by the plastic they have been rubbing against.

 

Yeah after these trucks hit 150K it would be good cheap insurance to replace the vac and water lines.....
you know the ones that get cooked by the engine.

It helps to have some color besides black for easy future inspection.....

Also allows rerouting some of the hoses coming off iac, so everything is not a tangled mess around fuel rail and injector harness....

 

What are the best places to source various sizes of new colored silicone tubing from? I'd like to eventually do this to my 3.4 even though my original hoses are fine and they've all been recently manipulated during my VC job. I do however have new OEM upper and lower rad hoses.

 

I started with the water hoses 1st.....easy to find those...
as for the vac hoses.....I'd just order few feet off ebay in couple diff sizes.....these are ordered by I.D. , so O.D. or how they look can be deceptive.......and then figured how to scale up or down size and footage needed and then ordered more from there....bag of multi size clamps...

The left over pieces are highly sought after, they make the ultimate split loom for when you want to insulate those old wires all around the engine....all my wires now have something around them....ones closest to the headers are double insulated.... even the throttle/tranny cables....so you dont cook the gease out of them......

 

Only exception was PCV valve line, that one needs to be fuel resist....
So I had to buy a 10 ft coil of largest dia of snowmobile fuel line I could find.....for like $8...