99 Silverado Rough Start
 

I’m new here and have reviewed old threads, but still can’t solve my problem. Any help would be greatly appreciated. I have a 99 Silverado 1500 4x4 with 5.3 and 227K miles. I’ve owned it since new and maintained it well. Recently it requires a little more cranking and stumbles briefly on cold (Wisconsin winter) starts. It has always started immediately with very little cranking required. Now I have to crank it 1 or 2 seconds before it starts and then it stumbles and runs rough for 1 or 2 seconds before it smoothes out. I sometimes have to try 2 or 3 times before it catches, other times it starts fine. Not a big deal, except that it will probably get worse, so I would like to fix it now. Idles and runs good after it starts and smoothes out with no error codes and mileage is good at 17 mpg.

The engine is bone stock. I use only OEM parts (air filter, plugs, etc.). The engine sensors and controls (MAP, MAF, IAC, TPS, injectors, O2 Sensors, EGR, etc) are all original. I have replaced the plugs and wires every 100K miles. Intake gaskets were changed at 100K when I replaced a noisy lifter. Fuel pump and sender were replaced at 150K.

I first noticed the problem after I cleaned the throttle body with TB cleaner. I do this about once a year as deposits cause it to be a little sticky when the throttle is first depressed. Can’t say if it started right after I cleaned the throttle, but soon after. It was also getting pretty cold about that time, so it may just be a coincidence.

So far I've tried doing a water spray test on the intake right after a cold start. Tried it several mornings. No steam out the tail pipe, so intake gaskets seem good.

I thought the problem may be caused by low fuel pressure and that after the pump ran a few seconds, the pressure built up and it smoothed things out. I tried turning the key directly from OFF to Start without pausing in the ON position and compared that with keeping the key in the ON position for 5 seconds before cranking and starting. No difference detected. I don’t think it’s low fuel pressure due to the test results and the fact that the pump was replaced at 150K. I haven’t measured fuel pressure yet, but could do it.

I tried several tankfulls of mid-grade gas (usually use regular) and bought gas at different stations. No difference, so I don't think it's bad fuel.

The injectors have 227K on them, but the MPG is good and it runs fine and idles good after it starts. I’m thinking injectors are ok.

I’ve noticed that it seems to start and run better if I depress the accelerator slightly before I turn the key. I’m thinking that the IAC may be bad or the throttle stop, set screw may need to be adjusted. I have a GM manual and it doesn’t give a spec for throttle plate to throttle body clearance. I know it comes from the factory preset and it’s not to be changed. Can the clearance change over time? Can the IAC stick and not work right at startup, but work fine once the engine is running? How about the TPS?

Any thoughts, comments or suggestions would be greatly appreciated.

Thanks

Re: 99 Silverado Rough Start
 

When you clean your throttlebody are you removing it from the plenum or just spraying it down with it still attached? It might not hurt to pull the throttle body off the plenum and thoroughly clean it from the backside as well. You should be able to clean out the iac passage this way too. Just 3 little bolts/studs, and the 2 little coolant lines and its off.

Re: 99 Silverado Rough Start
 

I just remove the intake duct and spray cleaner in and use a paper shop towel to clean it out. This always fixes the sticky throttle problem. You're right, though, there could be crud on the back side that I'm not getting out. I'll pull the TB and clean it and the IAC port when I get time. Thanks!

Re: 99 Silverado Rough Start
 

umm try spraying come carb cleaner around the intake. could be a temp sensor going out

Re: 99 Silverado Rough Start
 

You mean to check for intake gasket leak? Shouldn't the water spray bottle test do the same thing?

Re: 99 Silverado Rough Start
 

not really carb cleaner will make it stumble ,as where water if dont suck alot of it it will run fine

Re: 99 Silverado Rough Start
 

I'll give it a try. Thanks

Re: 99 Silverado Rough Start
 

it may sound stupid but pull of dist. cap and check for deposits

Re: 99 Silverado Rough Start
 

No dist on 99-up NBS/Gen III.

Re: 99 Silverado Rough Start
 

Coolant does not flow through the intake on the 99-up Gen III small blocks. Intake failures on these engines are usually a damaged gasket or warped intake that allows air to enter and make the fuel to air ratio lean. If bad enough it will set the SES light for the O2 sensor/s on the bank effected. This can also cause some bog when the pedal is depressed.

I would definately check fuel pressure. Also check for a failing fuel pressure regulator.

I wouldn't adjust the throttle stop. Low fuel pressure could possibly cause inadequate spray at the injectors and the extra air from the slightly opened throttle body helps to blend the dripping fuel.

Re: 99 Silverado Rough Start
 

so I guess this might be a silly question, have you tried any winter fuel additives?

or any additives for that matter?

your problem sounds very similar to mine awhile back with my '05.

I used my tuner and turned the idle up 150 RPM's and it runs/starts great now.

I would look into the fuel pump though, if I leave my key on "on" and then to start, it jumps up to damn near 1700 RPM's before settling back to high idle.

otherwise it's around the 13-1500 RPM range. I haven't tried this yet, but maybe someone can instruct you on introducing sea foam into a vacuum line. this supposedly does amazing things?

keep us posted!

Re: 99 Silverado Rough Start
 

Thanks, all good suggestions. I used a water spray bottle to spray water on the intake gasket area. I've seen this before and if there's a leak, you will see a big cloud of steam out the exhaust. I also tried the TB cleaner yesterday at the intake gaskets. No change in idle. I also bought a TB gasket and new coolant hoses for the TB. This weekend I'll pull it off and clean the TB and IAC. I'll measure fuel pressure too. I haven't tried any fuel additives yet, but may try them after I clean the TB. I guess I could use a tuner to raise the idle, but it has been fine for over 200k, so I'm thinking it's probably something else.

Re: 99 Silverado Rough Start
 

Sounds like a possible injector leaking down. Would explain the hard start and the rough idle for a few seconds. Leave the fuel pressure gauge on the rail, after the truck runs for a minute, cut the truck off and see how long it takes to loose fuel pressure. You can do a search to see how long it's supposed to hold fuel pressure.

Re: 99 Silverado Rough Start
 

I'll check the fuel pressure, but I don't think it's leaking down. That's why I did the test where I turned the key immediately to start and compared it to leaving it in the ON position for 5 sec or more. No difference noted. The 5 sec should have been enough time to build the pressure up. I'll pull the TB and clean it and check fuel pressure this weekend when it warms up. Been below 0 all day today and gonna get really cold tonight.

Re: 99 Silverado Rough Start
 

The pump only runs for two seconds unless the truck is running. It sounds like a injector leaking down or a fuel pressure regulator to me. Sounds like its flooding itself and that is why it takes longer to start. Intake leaks on these trucks usually cause a loping idle when first started and fuel injector/regulator problems usually do the rough idle thing.

As the others suggested hook up a pressure gauge and see if it'll hold pressure for more than 5 minutes or so. Also pull the vacuum line off the regulator and turn the key on and/or start the truck n see if gas sprays or runs out. It wouldn't hurt to clean the injectors either.

Josh

Re: 99 Silverado Rough Start
 

Thanks, I'll do that.

Re: 99 Silverado Rough Start
 

Had similar symptoms on two 5.3's. One was the fuel pressure regulator, the other was leaking intake gaskets. The intake wasn't bad enough to suck in carb cleaner and was worse when cold. It starts much quicker now with new intake gaskets. IF this is the problem, get the revised TEAL gaskets from GM only. They are expensive but fix the problem. Others I've heard don't do so well.

Re: 99 Silverado Rough Start
 

Good information. I'll keep it in mind. Today I'll clean the TB and IAC and check fuel press and see what I get. Thanks.

Re: 99 Silverado Rough Start
 

Pulled the TB and cleaned the back side of the throttle plate and the IAC passage and control. Lots of crud on everything. Tried to check fuel press, but my gauge kit is missing the GM adapter. Starts and idles fine now, even after sitting overnight, but it didn't start hard on every try before, so it doesn't mean much. I still need to get the fuel press gauge on it. Cleaning the TB was way too easy. Things are never that simple in life. This is a great site. Thanks to all who posted replies. I will keep you posted on the progress.

Re: 99 Silverado Rough Start
 

Truck's been starting and idling fine all week. Plenty of cold weather too with temps this morning below 0. Finally checked fuel press and got just under 60 psi and it held press well, even overnight. This fix (cleaning the TB) seems way too easy, but maybe I got lucky. If the problem returns, I'll try the other suggestions posted. Thanks again for all your help.

Re: 99 Silverado Rough Start
 

I had the same problem and it was a bad fuel pressure regulator.

Re: 99 Silverado Rough Start
 

This post is long overdue. Turns out it was not the TB. Problem returned after the outside temp warmed up. It was real cold (single digits) when I cleaned the TB but when the temp got back up in the 30's the problem returned. Let it sit 2 hours after running and put a fuel pressure gauge on it and had 0 psi. Pulled the vacuum line and gas ran out. Put in a new press reg and it's been running fine now for 2 months. Problem solved.