98 gmc 1500 5.7 rough idle for 20 seconds at startup

[rockmond]

OK., been searching for answers over the internet for this.

Fuel pump and regulator replaced.

dist. cap, rotor and plugs.,

tbi taken off and iac replaced all gunk cleaned.

tps replaced.

map sensor replaced.,

doesn't appear to have any vacuum leaks.., pcv valve seems to be working.,

 

local shop wasted some money chasing the problem and has thrown their hands up in the air..,

 

115,000 original miles

When first started, motor runs a little rough., if you try to immediately take off it will miss and buck ...,

If you just give it 20 seconds or so it will smooth out and then run fine no misses..,

I am tired of throwing money at it.,

 

I am wanting to sell the truck but I don't want to try to sell it with this problem.., I won't even get my recent money out of it.., wah..,, and wah..,

 

any help would be much appreciated.,

[ifixedit]

Not any way I know of checking the spider injection arrangement without tearing the intake apart. How about throwing a couple bottles of that Chevron injector cleaner in the tank first and see if that does anything. The reason I mention the Chevron stuff is I was told it's the same the GM dealers sell for like $30 a bottle and at the auto parts store the Chevron stuff is like $4.99!

[ifixedit]

Excellent! Glad to hear you got it fixed and running good! I always say, it's great when a plan comes together.

[rockmond]

Thanks for the replies..,

 

Fuel filter was also changed..,

 

I had tried the battery cable disconnect but didn't touch them together.., I will try that today.

 

 

[rockmond]

Well since I gave up on all other options.., I bought a new computer which is pre-programmed vehicle specific to the VIN.. and service# of the existing computer.., will get it in next week and will see where this all goes.

 

I will also check some of the other items..,

 

Don't get a check light at this point.., so no codes.,

 

The shop that had it for a week said they were getting misfire codes and as they replaced part after part troubleshooting method.., still had the problem., fortunately they just put my existing parts back on as they proved OK., but they never got it fixed.

[ifixedit]

The only other guess I would have is something with the fuel injection being partially plugged or is not delivering a nice mist on a cylinder or two and instead the fuel is coming out in droplets which might explain the occasional misses when it's been running awhile and also the rough idle on cold startup as this condition it would seem the truck would have to run a few before the issue clears up to an extent. You're basically getting a very rich cold start condition where the fuel is not atomizing.

 

I have a 98 Yukon with the same set-up so your truck I believe also has that interesting "spider" fuel injection arrangement, does it not?

[rockmond]

Well new computer didn't fix anything., figured I would just throw in a $14 temp sensor., kicks tire.., didn't fix it.,

 

Is there a way to verify/check spider injector? or any other ideas?

 

[ifixedit]

Since you were in the fuel system, did you also change the in-line filter under the drivers side door on the frame? Is the check engine light on?

[oldscsc]

After your pickup sits overnight, get a scantool on it and check the coolant temp sensor.

 

If the sensor has a dead spot (inop. during certain temp range) the truck's A/F ratio can be way off and cause a misfire until the coolant warms up enough for the faulty sensor to begin reading correctly.

 

Don't just throw a sensor at it. From a cold start, watch the digital readout of coolant temp on the scantool and monitor. Any big, sudden, or obviously incorrect (o.e. -40℉) will indicate a faulty sensor.

[Colossus]

when did it start doing this, and did it happen gradually or did it all start happening at once?

 

Another thing I'd like to suggest is that you disconnect the battery cables for about 30 min.. touch the battery cables together (disconnected of course) then re-connect, fire it up, let it sit ofr about a minute or so and then drive it around for about 15-20 min and see if that "semi-reset" of the onboard computer helped matters. Your heated o2 sensors may also be getting tired. You mentioned cap, roter, and plugs, did you also get the plug wires? Do a good check of as many of the exposed wires as you can to check for tightness. How's the battery and alternator doing? replace the PVC valve anyway. Looks mean nothing. Also time to consider a good motorvac on the 350.

 

Is it only acting up at the first start up of the day? Always a good idea to let the engine warm up for a minimum of 30 seconds before taking off when it is cold. It allows the oils and fluids to circulate a bit. Cold fluid/oil is very thick, even synthetics which means higher pressure which means more work for the componants/accessories.

 

We'll get this figured out.

[rockmond]

So..., just replaced the spider injector as a complete assembly......., drum roll......., runs like new..!!!!!!! Thanks for the advice. This has been a pesky little critter.., but it is over with.., for now