Need Some Tech Opinions

[jro909]

I'm at a loss, i've been having this on and off issue with my rpms dropping below idle and the motor almost killing, i thought it was the air intake tube and tightened it and the problem went away for a week or so...well i drove the truck 3 times today for a total of 50 miles or so and it was flawless the first two and the third it did the rpm issue a few times...heres what i can gather as far as triggers

 

1) seems to do it when not fully at operating temp

2) only does it if i feather the gas pedal enough to coast around in 1st, then suddenly let off the gas pedal

3) it does not die, it just goes below idle hesitates and bounces back up to idle speed

4) idles fine, runs fines on highways and around town

5) its not always constant, its quite random and i cannot duplicate it at will

 

thanks in advance for any possible solutions or causes, i know a few other guys are having this issue also, but like i said if i'm just slightly pushing the gas pedal to keep the truck coasting then suddenly let off it will do this, not always though i just want some opinions or solutions before i start barfing up money for diag fees

[Witiku]

I would get a can of starter spray or WD40 and spray it arround the engine untill I found the leak It sounds like a leak in the intake gasket system. I THINK thats a symptom of the 'O' rings that are part of the intake gaskets intermittently failing.(IYDNK The engine will idle up when the hydrocarbon gets sucked into the engine through the leak)

Best,

W

[ColoradoSkiCountry]

Do a throttle body cleaning first. That and cleaning my MAF Sensor solved my rough idle issues on my old 99 5.3L

[jro909]

why would it be so random? i'm sure intake gaskets are going to be a nice chunk of change to get put in any other opinions? woulden't there be some sort of noise? the truck diden't do it today at all, ran fine...i'm at a loss

 

what are the symptoms of intake gasket failure?

[Witiku]

why would it be so random? i'm sure intake gaskets are going to be a nice chunk of change to get put in any other opinions? woulden't there be some sort of noise? the truck diden't do it today at all, ran fine...i'm at a loss

 

what are the symptoms of intake gasket failure?

 

You have the symptoms of that failure, no noise, that'd indicate a mechanical failure ie TB gears etc. An engine is just an air pump, you are likely encountering a confused air fuel mixture causing an intermittent lean condition and rough operation. No big failure.... IF that the source of your low idle....

 

It's not a big repair, just has many steps that can't be messed up..., you can do it but it but a shade tree mech is a good option. Autozone parts and one hour, about $100 as I recall for my 5.3L Avalanche. The randomness has to do with the temp and pressure of the air on the running engine. This prob pops up big time the first cold day of the year. The engine runs ok with warm air seeping in but the cold air messes with the sensibilities of the engine and it idles to a stall or runs like it has a broken rod. Once hot the heat lets the parts 'melt' together and seals up better it runs great.

 

You said you tightened the inlet tube and that helped. This indicates you are on the right path, IF your co-relation is accurate.

 

Spray little puffs of starter fluid around the intake and TB assembly when it is running poorly and listen for acceleration, There would be your leak. You should also go to Autozone and get a can or Mas Air Sensor Cleaner ($7) and spray the elements carefully, once again you are looking to clean up what the engine thinks its pulling in, see?.

 

Relax and look for the problem, shouldn't be too $$

 

Hope you find the issue and it gets fixed...

[jro909]

the loss of engine revolutions only happens when moving or stopping, i can idle the truck with zero problems

[cwendel4]

don't know if your model has this, but check the fuel pressure regulator. Pull the vacuum line off of it just after shutting off the engine. If fuel is in the tube, you have the problem. Had this on my 00 when I got it. Idled fine but bogged like crazy for no reason.

 

Also think about changing your fuel filter. Gives than same types of problems you are listing.

[minibike132]

why would it be so random?

 

 

Alright I'll give it a shot. This might sound a bit abstract to anyone that doesn't have good working knowledge of how an engine works plus the hows and whys of what the PCM is doing.

 

When the engine is cold it runs in open loop which is a pre calculated air fuel ratio and spark timing based on only a few of the inputs to the PCM. Some of these are air intake temp, coolant temp, throttle position, mass air flow, manifold air pressure. Once warm the engine starts watching feedback data, the most critical of which on this vehicle is the pre cat O2s. The PCM watches the O2 data and adds or removes fuel based on the rich/lean status to adjust the engine to optimum conditions. Once an engine is warm there are mechanical things that effect running and in a simple engine this is all you have to look at. With a computer controlled system you have the mechanical items but you also have the PCM helping the engine achive proper operation. This fools you into thinking the problem isn't there. The reality is that the problem is there but it is being corrected so quickly that you don't notice a thing.

 

Ok now why will a vacuum leak do this cold and not warm. Mechanical reason is that seals are slightly smaller and not as pliable when cold. Any seal that leaks will likely do it more often when cold. The electronic reason is that the PCM is running a program for a normal running vehicle when cold and fast correcting when warm.

 

What you are describing sounds like a lean condition. What causes lean conditions? Lack of fuel or excess air. If you said that you hit the gas and this happens I would lean toward fuel delivery, you touch the pedal the engine wants more fuel. If it isn't there you get a stumble. Now when you say that you let off the gas and this happens we lean toward the vacuum leak side of this. This is becuse when you have the throttle open that is a vacuum leak, wide open would be a large calculated vacuum leak. When you close the throttle if there is a vacuum leak then there is a small uncalculated amount of air entering the engine (that makes it lean). Since the PCM is ignoring the O2s correcting for this is slow enough to be noticed.

 

So throw money at it and hope the guess sticks or pay someone to get into it. With some live data on the scan tool you can force the engine to react to an outside fuel source if you suspect a vacuum leak. You can watch the reaction of the O2s untill you hit the spot of the leak. You can see in the numbers what you can't feel when the engine is warm because you can see the corrections the PCM is performing while you introduce the outside influence. With guesses you get just that, my educated one is that if the MAF is reporting correctly you have a vacuum leak. If you have a vacuum leak my money is on the intake based on odds. It could really be anything though, you are not going to get anything other then a lucky guess without some hard data on this one IMO.

[cwendel4]

Hey, sparkstech, it just hit me after reading your post. It has been a long time, but my wife's old nissan had this issue when the A/C was on. It turned out to be a very dirty Idle Air Control Valve. It was caked up around the plunger and since the valve moves very little the dirt was obstructing the correct amount of air to enter the throttle body. No codes were set because the plunger still moved but the dirt was stopping the flow of air. The IAC valve controls the amount of air that enters around the throttle body when the throttle blades are closed to regulate the engine idle speed. That is why we can not set idle speed the old fashioned way. And yes, this still applies to when the truck is moving and the throttle is closed

 

You can try a quick and dirty clean by spraying throttle body cleaner into the small opening at the top of the throttle body. This is the IAC passage which leads to the valve. You do have to take the intake elbow off to get to it. Try this first because if it helps at all then you will need to clean the IAC valve.

 

 

If the truck still has a mechanical throttle body and throttle cable, the IAC valve is on the opposite side of the throttle lever. It is a big round cylinder held in with very tiny torx screws, so be careful when removing and replacing them.

 

Now to clean it. Take some throttle body cleaner, NOT CARB CLEANER and spry just the plunger end of the valve enough to clean off the deposits. Do not spray the rubber o-ring that is on the valve, if you do it WILL SWELL and you will not get it back in without tearing it and you have to find a replacement which can be very difficult if you do not have a supply of o-rings. Take a soft rage and gently brush off any dirt from the plunger, do not push in or pull out on the plunger or you will have to buy a new one. Then clean the inside of the opening in the throttle body with the same spray to get rid of all the carbon and dirt that has built up over time. Regular throttle body cleaning will not clean this out and is only sometimes done when the TB is removed for cleaning.

 

This has fixed many odd problems. Remember folks, this is the computers way of controling air flow, just like we used to do with drilling holes in carb butterflies.

 

Hey if anybody can come up with better ideas, let me know.

[jro909]

the truck is drive by wire, it does seem a little sluggish on the accerleration sometimes..like it hesitates...i'll have the intake gaskets checked into, if it is fuel related i'm kind of boned seeing as the system isen't serviceable..the fuel filter and i think the regulator are all in the tank..one thing i noticed is that it becomes quite a terd after driving in stop and go traffic with the a/c on in hot weather like i said i've never cleaned the maf sensor or the throttle body the truck has 110k on it

[jro909]

hasen't been doing it lately, the temps have dropped along with the humidity so i'm not sure if maybe that could of been the cause i'm trying to gather as much information or possible causes before i go in and pay for a diag