99 Bravada stumble on startup

[YooperJ]

This is related to my poor running problem in my 1999 Bravada. It runs rough and backfires upon acceleration for approx 30 seconds and then is fine. It does this on any startup, any engine temp and any ambient temp. I'm thinking there must be a timer in the computer that is making it run (lean?) bad for 30 sec. I can't think of anything else that would do this for a specific time period under different operating parameters. Anybody hear of anything like this or am I missing something obvious. No trouble codes, ECT, IAT, MAF, MAP all seem to be within specs. Converter has no back pressure buildup, under .5 psi. AC spark plugs changed along with air filter. Fuel pump is within specs. This started happening in the fall of 2009.
Now it's March 2011 and I still have the problem. Approx 125K on the vehicle and since the lst post I changed the Fuel injector system to the new one and replaced the Idle Air Control solenoid. Still have the problem. It only happens for about the first 30 seconds so I'm thinking something to do with a timer or something that depends on a timer. I changed the main Computer with a good one and went through the security setup, same problem continued. I figured the timer part had to be in the computer so that's why I changed it. I talked to several mechanic friends and they would have done the same thing. No engine codes or service engine lights other than a temporary one with the MAF.
So now I figure it must be something that is different for 30 secs after startup that is the problem. Fuel delivery on startup requires more fuel so I checked the fuel pump. Fuel pump pressure was a bit low, 55 psi running so I had the pump repalced with a new A/C Delco one along with a fuel filter and air filter. Same problem.
What now? Could it be the transmission? My son had a Murano that had a similar situation but not related to the time element that drove the Nissan dealer nuts. Changed the cam sensor, Throttle body and intake and still had the problem. Finally changed the transmission and problem went away, car was tied up for 2 months because it was too new to get parts. MY car stumbles bad no matter if I start off in 1st gear or drive so it's not a shift problem or gear problem, but just what could it be?
Any help or guidance would be greatly appreciated and I hate to beat an old horse dead but there has to be a logical reason for this.

[Rick Norwood]

What size, engine 4.3L? It seems like you've spent a lot of money on parts, did you replace Wires, Cap, and Rotor? Is your Distributor Cap a flat top style or a birthday cake style?

The reason I ask is that your problem sounds somewhat like mine.

See this thread. http://www.automotiveforums.com/vbul...d.php?t=975186

[YooperJ]

I have the birthday style. Did not change those items as they look good and it only happens for the first 40 seconds or so after startup. After the short time interval it is better, takes off from a stop with lots of power. My wife mentioned that at high speed when trying to pass, she floored it and it hesistated. Starting to think transmission???? Oh bye the way, I am GOING crazy over this.

[viggy58]

even a good looking cap can have a tiny crack in it allowing moisture in. it will stumble until the spark essentially burns off the moisture. Plugs, wires, and caps are always the best place to start. use ac delco parts and make sure the plugs are gapped correctly.

[blazee]

Originally Posted by YooperJ

I have the birthday style.

Yours shouldn't have the birthday cake style (wires enter through the top of the cap). Your 99 should have the horizontal cap, where the wires connect to the sides of the cap.

[Rick Norwood]

Viggy58 is dead on right. Even a nearly new cap can be a piece of SH#$ if it is not an AC-Delco. My AutoZone Duralast Cap went to hell in 6 months in my 2000 Jimmy (which is essentially the same set-up that you have in your 99), and I assumed since it was brand new that it simply COULD NOT BE THE PROBLEM, but it was!

If your cap is the original OEM cap (which it doesn't sound like it is if Blazee is right), or if it is just plain old, Go buy an AC-Delco Cap, Rotor and Wires, and you'll probably solve your problem. Yes you will spend close to $100 bux, but it is worth your sanity.

And BTW, my 2000 Jimmy has a Flat-Top cap not a Birthday cake cap. I think Blazee is right. Check with your parts store and BE DAMN SURE YOU GET AN AC-DELCO CAP.

[YooperJ]

Sorry, I do have the pancake style. I was thinking of my truck instead. Still have the problem, and thinking it may be trans connected. Anybody know where I can find out info on the Transmission operation?

[Rick Norwood]

I really don't see a connection between backfiring and a transmission. Backfiring and those types of issues are secondary ignition issues. If you pay someone to do a tranny scan, you'll probably be wasting your money.

Chalk one up to routine maintenance and swap out the cap, rotor and wires. Just make sure they are all AC-Delco parts.

[YooperJ]

Changed the cap and rotor with A/C Delco parts when the vehicle wouldn't start after a rain day. The new cap & rotor look a lot cheaper than the original ones and didn't say AC Delco anywhere other than the box. Still have the 30 second problem with the stumbling upon acceleration from a startup. Just about convinced to put a note in the car that says "Wait 1 minute before moving". Any other ideas??? Am I nuts or WHAT

[YooperJ]

Just a follow up on the Bravada. I got a MAF trouble code so I took it apart and cleaned the 3 wires which were very dirty. Car now runs like a dream!!! This was my problem from the get go. From now on, the MAF is going to be cleaned at regular intervals on all my vehicles. Thanks to all that posted suggestions.