solving rough idle on 1993 silverado 5.7?

[rhastings80]

I have a 1993 GMC C1500 rear wheel drive pickup with a 5.7 liter gas engine TBI. The truck has 104,000 miles. The truck runs pretty good but when the truck is in park and running sometimes it is quite rough. Also when I’m driving at times when I’m stopped in drive at a stop light it has a rough idle as well and every 10-20 seconds or so it will make the truck shake. I also notice when this happens the RPM’s on the tack jump around a bit. When I"m driving the truck runs smooth and seems fine.

Does anybody have any idea’s what the problem may be?

My truck was way overdue for a tune up and I have performed the following but it hasn’t helped much:
Changed spark plugs with OEM AC Delco 30k plugs and gapped them to 0.035
Replaced spark plug wires with AC Delco plugs
New AC Delco distributor cap
New distributor rotor
New AC Delco fuel filter
New aftermarket pcv valve
New AC Delco air filter

After replacing the stuff above I went to my mechanic as I didn’t want to just throw money at the problem. He suggested that I take off and clean the EGR valve as he said these truck build up a lot of carbon. I decided to just purchase a new EGR valve and solenoid as they were not all that expense.

The truck still idles rough at times in park and when stopped in gear in drive.

Does anybody have any suggestions of what could be wrong. The truck doesn’t have an OBDII connection.

Thanks!

06/18/2010 Update:
Hi Everyone,

Sorry it has taken me a while to respond. I have been busy with work.

Anyways I replaced idle air control valve on my truck as the old one looked original and was totally caked in carbon. This didn't really seem to help though. Do you guys have any other ideas? I looked at all of the vacuum hoses but they all seemed to be on tight and didn't see any cracks. I don't have a smoke machine though.

I may have to bring my truck in to have somebody look at it as this may be above my skill level at this point.

 

[jay50]

Be sure to check the map sensor vac hoses to ensure they are secure to the sensor as well as the vac ****** on the backside of the throttle body. Have seen a few of these where the hose at the TB was knocked loose and caused idle issues.

Also might want to removed the IAC to clean out any crude that could be blocking the port into the tbody. Do not attempt to clean the IAC in any cleaner.

 

[nehog]

Seconds on the IAC, they are cheap, just replace it if you have to.

 

[737mechanic]

IAC should be the first thing to look at. Make sure all the vacuum hoses are in good shape by pinching around on them and see if they feel spongy or crack and break, if they do then replace all of them. Also make sure the throttlebody bolts are tight. If that doesn't help then pull the throttle body off and clean out all the blocked ports, most of the time you can even save and reuse the gasket if it looks good.

 

[bgott]

I wouldn't re-use the throttle body gasket. The gasket on these engines had a propensity to burn out at the rear of the TB, leading to a high idle. The old gasket has aged and re-using it will weaken it.

 

[Keep]

Buy a bottle of sea foam. Follow the directions, enjoy a better running truck.

Oh and check the o2 sensor as well.

 

[LT4]

Check the battery and the charging charging system to include the belt.

 

[930dreamer]

I have a similar issue on my 91 Suburban 350 TBI. The first thing I looked at was to see if the TB base gasket was sucked in at any corner. My wasn't so back to the drawing board.

 

[Gearhead559]

clean the throttle body, change the fuel filter, and i have seen on these, the distributor gear likes to wear out and cause odd issues from time to time

 

[rodm1]

Are you getting a oily film coating inside the TBI? Is it using lots of oil? If it all sow hunts for idle when cold and retarding the timing helps (if I remember) It might be worth having a buddy check the intake valley gasket if it has one.

I had an 1987 that ran like above. Since going to a Dodge I've found out that they have huge problem with there plenum gaskets. I always wondered if replacing it on my Chevy would have fix it. It's just a guess.

 

[rhastings80]

Hi,

Yeah it does seem to use oil as I find myself having to add more between oil changes. I will have to check in the morning but I think there may be a oily film in there. I was guessing that was normal?

Thanks

Are you getting a oily film coating inside the TBI? Is it using lots of oil?

 

[finn]

My neighbor just bought a similar Chevy at an auction...same rough idle issue (~110000mi)

Cap, plugs, wires etc. were new. After alot of head scratching and several dead ends (including replacing an injector or two), resetting the timing to spec smooothed it out perfectly.

I'd suspect a timing chain or worn distributor gear is what let the timing retard, but his runs fine now so he's happy.

Alan

 

[MattT]

The truck doesn’t have an OBDII connection.

Might have OBDI if it's anything like Fords of similar vintage. If you do pull a code on it check it's really the sensor, or whatever, that's defective before buying a replacement. Might just be a bad connection especially on a vehicle of that age.

 

[MechanicNamedJohn]

Sounds like a vacuum leak to me.

 

[Tarheelgarage]

Might have OBDI if it's anything like Fords of similar vintage. If you do pull a code on it check it's really the sensor, or whatever, that's defective before buying a replacement. Might just be a bad connection especially on a vehicle of that age.

Yeah, use the old paperclip and get a flash code. Those ECMs for OBD1 from 93-95 with TBI had a good bit of datastream with the proper scanner adapters. Showed fuel trims,load, 02 voltage, etc.

How does it idle at cold startup? Does it seem to idle better when engine is cold vs when hot at operating temp?

 

[bgott]

clean the throttle body, change the fuel filter, and i have seen on these, the distributor gear likes to wear out and cause odd issues from time to time

If it needs a gear I would buy a rebuilt distributor. The magnet in the shaft likes to crack and give problems and you can't just buy the shaft anymore.

 

[ddawg16]

I have the exact same engine in my jeep. I'm chasing down a similar issue....but not exactly the same as yours. You really need a OBDI scanner to see a lot of the data.

How is the gas mileage?

Tarheel asked good questions....what does it do when cold?

What do the plugs look like?

How much oil is it using? A quart evey 3K miles is nothing to worry about....a qt every 1000 miles is something to start worrying about.

What is the vacuum? You can actually troubleshoot quite a few problems just with manifold vacuum.

In my case, I'm tracing down why my timing jumps every time I get my stumble. When I'm watching the timing advance on my scanner...the timing will jump from about 16 deg to about 22 deg when I get the stumble....the problem is I don't know if the jump is in reaction to the stumble or causing the stumble....

 

[judgethis]

When you changed the EGR valve did you clean out the port? It may look good at the EGR but usually clogged where it mates to the head. Stick a coat hanger in the port and see if its clogged. Then start the engine with the EGR off and it should sound like a very loud exhaust leak. Rev the engine really quick a few times to really clean it out.

 

[jethro29]

sounds like you have a vacum leak to me , you can check with carb spray,buy a smoke machine is your best bet,pay close attention to the area around themanifold and look for signs of leaaking oil or coolant.

 

[Tarheelgarage]

When you changed the EGR valve did you clean out the port? It may look good at the EGR but usually clogged where it mates to the head. Stick a coat hanger in the port and see if its clogged. Then start the engine with the EGR off and it should sound like a very loud exhaust leak. Rev the engine really quick a few times to really clean it out.

I've gone as far as making a small metal cover and removing egr and blocking the hole shut to see if problem is with egr.
Note that GM OBD 1 did not do a good job at throwing codes for lazy 02, map, or other sensor. The code set for open/short circuit conditions, not for shewed/out of range conditions. Don't overlook the possiblity of the engine coolant temp being skewed and causing this condition. It has its fingers in all of the powertrain management activity. Here again, a scanner with OBD1 capability would allow you to review the datastream PIDS to quickly give you the straight story as to what is being reported to the PCM.

When truck is idling rough, do you see darker exhaust appearance? If so, you have a fuel enrichment condition for some reason.

About 6 months ago, had a 95 Chevy TBI come in with same condition and found the owner had knocked the map sensor hose at Tbody loose when he installed a new EGR.

Head down south to my shop and I'll be glad to fix'er for yah....

Tarheelgarage...
Being turned over soon to the next generation of spark chasing wrench men

 

[Tarheelgarage]

sounds like you have a vacum leak to me , you can check with carb spray,buy a smoke machine is your best bet,pay close attention to the area around themanifold and look for signs of leaaking oil or coolant.

I have a diagnostic smoke machine in my shop. Has been a life saver finding vac leaks that other shops have given up on.
You would be surprised at the small vac leaks found on most cars today that go undetected because the PCM is able to compensate by adjusting Long term fuel trim.
When I see LTFT go above 10 %, (+/_), there's an issue that should be addressed in most cases.

Tarheelgarge

 

[rhastings80]

Hi Everyone,

Sorry it has taken me a while to respond. I have been busy with work.

Anyways I replaced idle air control valve on my truck as the old one looked original and was totally caked in carbon. This didn't really seem to help though. Do you guys have any other ideas? I looked at all of the vacuum hoses but they all seemed to be on tight and didn't see any cracks. I don't have a smoke machine though.

I may have to bring my truck in to have somebody look at it as this may be above my skill level at this point.

 

[66HertzClone]

Do you have a propane torch? No, don't set it afire, rig a rubber hose with a metal tube to the torch. Use this to check for vacuum leaks, you can place the tip at the manifold gaskets and many other tight places. If there is a leak the idle speed will increase as the propane is burned.

 

[gatchel]

Does that year have the plastic intake. I have heard things about chevy plastic intakes cracking.

 

[slghmmr88]

I have been watching this thread to see what the op comes up with cause I also have a 93 5.7, don't have the same problem but always gathering info on what could be a future snag although with mine having 367K miles we are pretty far apart in mileage. And no I don't think this is a year with plastic manifolds cause mine looks to be aluminum since the paint has washed off maybe a couple of hundred thousand miles ago.

 

[930dreamer]

I had our mechanic look at my Suburban, the intake gasket was leaking, runs great now, smooth idle.

 

[jjkrjh]

If you haven't checked your distributor gear yet, you should. These engines are famous for the gear to "apple core" . Would also explain your tach bouncing around. Generally, if you have a vacuum leak the idle will be higher than normal.

 

[gearhead garage]

It is mst likely the spider injection unit. I have replaced so many on those year trucks. Usually one or more of the injector will leak fuel out causing it to run rough and miss. Since it is OBD 1 it won't show which cyl is misfiring on the scanner. Your best bet is to replace the whole unit. I actually have one that was installed on a 98 silverado with a 350 for only two weeks. I ended up replacing the engine, that is why I have the unit left over. These units go for 500 plus at the dealer and 300 plus rebuilt. Make sure that you also change plenum gaskets as well. All of the upper gaskets come in the kit for about $15.

 

[jay50]

It is mst likely the spider injection unit. I have replaced so many on those year trucks. Usually one or more of the injector will leak fuel out causing it to run rough and miss. Since it is OBD 1 it won't show which cyl is misfiring on the scanner. Your best bet is to replace the whole unit. I actually have one that was installed on a 98 silverado with a 350 for only two weeks. I ended up replacing the engine, that is why I have the unit left over. These units go for 500 plus at the dealer and 300 plus rebuilt. Make sure that you also change plenum gaskets as well. All of the upper gaskets come in the kit for about $15.

You must not wrench too much, huh.
This is TBI

 

[gearhead garage]

never mind, forget about my post. That would be for a later year. 93 is too early. Too many pain pills after my surgery.

 

[ddawg16]

You must not wrench too much, huh.
This is TBI

Ok....so I'm not the only one thinking that.....Spider injection is for the MPI...Vortec engine....

I'm really thinking we might have the same problem. I replaced my intake manifold gasket a couple of months ago....my vac gauge says good....but I did have a slight coolant leak at the front so I was thinking that maybe I was sucking in a little water around the #1 intake port....gasket made no difference to running....but the water leak is fixed.

I still think my issue is related to the timing.....I would like to think that my dist is causing the issue.....but....I can disconnect the timing connector (single wire gray connector on the wiring harness) and there is NO stumble...timing stays rock solid...(at 0TDC)

At note to others about vac leaks and high idle...that may be true with carbs....but on FI engines, the system will compensate up to a point for small vac leaks...only if it was a good size leak would the RPM's be higher.

Tarheel....you seem to have the most experience with the engines....what is the typical manifold vacuum in park and in gear with the engine warm?

Mine stays rock solid around 20" or so until the stumble...then it drops...but my timing also jumps....just can't tell if the timing change is causing the stumble or a result of it.

rhastings....you should really consider getting an OBDI scanner...they are pretty cheap....easy to eliminate the temp sending unit....monitor the value as it warms up...you can even tell when the t-stat opens....temp goes slightly above 195 then you see it drop down to about 180 or so as the cold water from the radiator is starting to circulate....once you see that happen....you know that your sending unit is pretty much working right.

Have you tried the wire wiggle? With it at an idle...wiggle all the wires.

 

[ddawg16]

Forgive me for hijacking this thread....

I fixed my timing light this afternoon (loose solder connection on the ckt board) and double checked my timing....I took the time to carefully watch the timing mark.....I think I can see about 2 deg of bounce in the timing....maybe more....hard to tell in daylight and the flash of the light at idle....and yes, the timing connector was unpluged so the computer was not trying to change the timing.

Is this normal? Or is my dist worn out?

 

[bgott]

How much slack do you have in your timing chain? You can always run a dial indicator on your distributor shaft and check the run-out. If you can move the distributor shaft back and forth you might have a bad distributor. I haven't seen a ton of worn out shafts on electronic distributors, on a points distributor, OTOH, a little bit of shaft wear would throw off your dwell and make it run like ****. I had a Dodge van that had a worn out bushing in the cam assembly. I had to set the points by driving it with the doghouse off , stopping and tweeking the adjustment until it would run right. I couldn't find a new points cam and I was being hard headed and didn't want to convert to electronic.

To the OP, how are your motor mounts? If the engine is grounded to the frame it can vibrate the fillings out of your teeth.

 

[metelburner]

I have heard of some of those 350's having the injector o-rings leaking . Just a thought ?

 

[dennism8224]

wow!!!!!!! I will try all your suggestions . Thanks for every ones help.WOW

 

[az45]

I had our mechanic look at my Suburban, the intake gasket was leaking, runs great now, smooth idle.

Im surprised this thread made it to page 2 without the intake gaskets being mentioned. Smoke test it, its pretty common.

 

[Mike~MIller]

From What I remember this problem comes up from time to time with this vintage of Engine, What I've found is that its the intake gasket at the rear of the engine has either deteriorated or more commonly the nasty ****** ethanolate fuels our kind and a Gentile government has force fed us has hoopajoopd it. so Wam her up good. let the idle settle then take whatever flammable libation you've got handy, (brake clean, seafoam, whatever.) and squirt it all around the intake if the idle changes and if you hear it-then you know its an intake gasket. I've fixed my share of Chevy 5.7s and 5.0s with this issue, seems it happens around the 100000-1500000 mark as long as its all is still relatively stock. Hope this helps but looks like others have chimed in already. So Ya check it out.

 

[texasprd]

An additional possibility is that the vacuum passage for the MAP sensor may be mostly plugged with crud inside the throttle body. That happened to me on my '88, which is the same TBI set up, and it drove me nuts trying to find it (turns out it is suggestion #2 in the symptom troubleshooting list on my SnapOn "brick" scanner). If the vacuum signal to the MAP sensor is restricted and therefore inaccurate, you can get some very annoying (and dangerous) drivability issues.

If you feel comfortable removing the TBI unit, take it off. On the underside (intake manifold side), between the throttle plates, is a small hole - about 1/8" diameter, IIRC. That is the vacuum source for the MAP sensor - it runs through the TBI unit to that port on the back where the vacuum hose connects. From the underside of the TBI, check that hole between the throttle plates for obstruction. Run a drill bit (smaller in size than the hole, of course) in there with your fingers and see if crud comes out. If so, get a can of air-intake cleaner and spray back and forth between that hole and the MAP sensor vacuum port on the back of the TBI unit.

There is possibly another way to detect this, but it involves multiple vacuum gauges, tees, etc.

Good luck!