random intermittent engine stumble...no codes

[NoLaTUNDRA]

Hi Guys. I'm back with another problem with my lovely 4.7L '02 Tundra (78k miles). I have an intermittent engine stumbling problem. It's been happening now for a month and never trips the check engine light. The symptom is a very rough stumble (5-10 sec) that feels like she's going to stall. The only common trait is that it happens at low RPMs...at initial start up or after decellerating from a higher RPM. The frequency of the problem is the kicker. It may do it once at start up today, and then run fine for two or three days and then catch a quick stumble while sitting in traffic. It's frequency is completely random but like I said, the only common trait is the lower RPM range.

Given my last stumbling problem over a year ago (water) I have been hesitant to add a fuel additive. I did last time and it took the dealership two days to identify what was in the tank.

This past week I changed the fuel filter and also poked around for a vacuum leak and did end up changing a line that runs from the evap canister to the intake manifold. It was really loose. Since I did both, there appeared to be an improvement but since I did both at the same time, I couldn't be sure what did the greater good. But that improvement last all of 4 days.

I inspected the PCV (1 year old) and it appears to be working...it rattles freely when shaken and simply blowing lightly in it moves the little valve.

I have been reading quite a bit about an apparent Toyota throttle body cleaning that just about every Toyota needs. I'll inspect the TB in the morning just to see if it is gummed up and dirty.

In the mean time... if anyone has any suggestions or a crystal ball I'd greatly appeciate them. No codes make it a nightmare.

Thanks again for any help you all can offer.

[MCoupe]

NLT -

I had very similar symptoms when one of my coil packs was going bad. But when my engine was stumbling, the CEL would flash, but not set. It rather quickly got worse, and started setting a light. I borrowed a code reader from a friend ($40 from Harbor Freight? Very basic) and found that one cylinder was "missing". Swapped that cylinder's coil with another coil, I cleared the CEL, then CEL came on and showed that the miss had followed the coil. Bing bang boom.

If you aren't getting a CEL, then your options are more limited. Idunno how you would test for a failing coil, must be a way, but probably costs more than one new mailorder coil. Particularly since you'd need to test 8. Which you could buy one and swap through your 8 cylinders until the problem goes away. IF this is your problem. . .

You could also leave it at the dealers, and have them connect a more expensive code reader to it, and drive it until the stumbling occurs. They would then have a good idea what is causing it. But if it is infrequent - hard (expensive) to pay for them to drive your truck for an hour. . .

Cleaning the MAS wire is a good idea, and it will undoubtedly be dirty.

I guess that I would call the dealership and chat with the service manager about finding a bad coil. Find out their plan, with prices, and make some decisions. I say call, as with your gasoline cleaner episode they obviously saw you coming - and it is time for new boats in Louisiana. . .

Good luck!

Chris

edit: oh, and search the forums here for testing methods for coil packs.

[NoLaTUNDRA]

I replaced the mass air flow sensor when the o2 sensors went out. Actually, 1st they insisted it was the mass air flow sensor, so they changed it. Well, after driving a mile from the dealership, all my troubles started happening again. So then they finally changed 3 o2 sensors (2 at their cost 1 at mine). SO given the fact they had codes to work with and still got it wrong, I have no plans to return to the dealer. We have a foreign car specialist here in this small town that I am new to, but I wonder just how much of a specialist he is. I may end up finding out.

Thanks for the info, after I get my throttle body back together I'll look into the coil pack issue. The only other thing I found while googling and burning the midnight oil was a faulty vehicle speed sensor...the author of the article said that when they fail, they fail intermittently and usually fail to produce an error code until it's finally dead. The symptoms he described are similar to mine ...stumbling during decelleration or idle because the computer doesn't get the correct speed information off the tranny and thus misses the corrections it needs to make to the engines fuel/air management.