consistantly inconsistant throttle issues

it seems like depending on which day of the week my 98 blazer seems to be tempramental. i'll start from what i think is the begining, when the problem started. jan 2010, changed water pump,next weakest link in the chain a week later radiator cap, two weeks later radiator, two days later heater hoses(and just on a side note after i changed the heater hoses i don't think my heat ever blew that hot in its 12 years of existance).Now, on friday i drove about 80 miles,no stop and go open counrty rodes about 40 to 60 mph, and everything was fine. at about 60 miles into my trip, i was going about 30 and slowed to about five to roll to a stop light. i down shifted(standard transmission)and as i did the throttle would drop from 2100 to about 1700 then jump to 22-2400 and stay open at about 2200rpms yet i was in neutral rolling to a stop. as i stopped to throttle dropped to about 1200 and jumped to 1600 then back to idle(bout 550). as i was at the light in neutral i gave it a little gas(three or four times not exceeding 1100) and nothing stuck, went up and back own to idle no prob. i shifted to 1st took off got back up to speed(about 40) and in fifth. as i came to another stop same thing, throttle stayed up dropped jumped and back to idle. this happened for bout 5 miles. i decided to go hang out and let the engine relax a little. turned off popped the hood looked things over seemed fine no leaks, no overheating no smells. an hour and a half later i started it let it warm up for about 15 min, and prceeded to drive home. about 20 miles later same thing. only this time the throttle would(when rolling to stop) didn't jump as much, maybe about 1500 it would stick and slowly drop back to idle. all day saturday, drove about 40 miles same driving conditions yet everything was fine. no throttle issues. on sunday the same thing that happened on friday happened.
i hope someone has an idea of whats going on. i don't think the "automotive plumbing" issues had anything to do with it i just figured it might be relavant. its a 98 5spd v6 vin W 4x4. i just need to know how to go about diagnosing this problem. i'm out of work for three mo. now so i have time to take things apart but no money to take it to someone unless absolutly necessary. i do plumb/heat/hvac so the water sys was pretty straight forward. the rest i need guidance so if anyone has any suggestions they would be greatly appreciated.

Any check engine lights or codes?

no codes no check engine light and after driving about 40 miles today, 20 miles had the same issues the other 20 though i switched the heat vent from dash vents to defroster and the throttle was fine. also the clutch fan on the a/c engages only in defrost(which i think it should).

so now i top of all these throttle issues the SES light finaly came on and i had the codes checked. 2 codes: p0420 came up twice. Now i know the code says "catalyst system efficiency below threshhold(bank1)", but what does that mean? and why would it come up twice as the only two codes?

How many miles on the vehicle? Have you ever replaced the oxygen sensors, there are 3 of them on your vehicle? Oxygen sensors react to changes in fuel mixture, both lean to rich and rich to lean. The faster the sensor can react, the better the engine performs. Oxygen sensor reaction time slows down during it's life. After 100,000 miles the reaction time has slowed significantly and they should be replaced. The P0420 DTC uses the oxygen sensors during the monitor and test. The PCM measures the time it takes for a given change in fuel mixture to reach the downstream oxygen sensor. If the sensor reaction time is slow, it can set the P0420.

DTC P0420, P0421, and P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicate bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicate bank 2-catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code is not present, go to next step. If misfire code is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC. Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test-drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.

WOW! MT that sounds like that should keep me busy for a while. thank you for the troubleshooting i will definetly get back to you with results. OM thank you too. theres 136k on my 98 blazer 4.3 "w" 5spd. also guys, i can't seem to locate the receipt for when it was exactley done but the cat.conv. has been replaced within the past 3yrs, and at least 20-24,000 miles ago.

You are welcome.
Good luck and let us know how it goes.

Did u find out what was causing ur inconsistant idiling problem? My s10 is idling high as if u r mashing the accel. especially when coming 2 a stop. However I am not getting any codes.

Did u find out what was causing ur inconsistant idiling problem? My s10 is idling high as if u r mashing the accel. especially when coming 2 a stop. However I am not getting any codes.

You (and the OP) I think have a vacuum leak. Search my posts for vacuum leak procedure, and follow the directions exactly as posted.

Vacuum leak will light no codes, until fouling of the cat occurs, which will come from idle loping, and unburned fuel getting into the cats and fouling them.

well jeremy so far i have found a vaccum leak,leaking intake man gasket, reamed out motor/engine mount, barely hanging on torsion bar support links, and something at one point or another was living between the intake man and distributor cap. is this the cause of my throttle issues...IDFK. i hope it is. in the process of dismantel/distroying this thing. vaccum hoses were all very worn so i'll be changing all i can. everything else seems ok. dist, cap, rotor, wires, plugs, tb, fi, ingn coil, sensors all bueno. MT and OM and CB i followed all your proceures thanks, you guys led me in right direction. BTW, anyone know why the second to last bolt hole on the intake manifold is open and the other 7 are closed?(second to last on drivers side from front to back)

98 blazer 4x4 5spd 136kmi vin W

well jeremy so far i have found a vaccum leak,leaking intake man gasket, reamed out motor/engine mount, barely hanging on torsion bar support links, and something at one point or another was living between the intake man and distributor cap. is this the cause of my throttle issues...IDFK. i hope it is. in the process of dismantel/distroying this thing. vaccum hoses were all very worn so i'll be changing all i can. everything else seems ok. dist, cap, rotor, wires, plugs, tb, fi, ingn coil, sensors all bueno. MT and OM and CB i followed all your proceures thanks, you guys led me in right direction. BTW, anyone know why the second to last bolt hole on the intake manifold is open and the other 7 are closed?(second to last on drivers side from front to back)

98 blazer 4x4 5spd 136kmi vin W

Open bolt hole could be a factory accessory that was not optioned on your car.




(like the supercharger option :grinno: )

alright intake manifold gasket and valve cover gaskets changed, so the mystery antifreeze leak is done and done. also changed two 3/16 vaccumm lines. no codes started right up a little rough at first but once up to temp it settled down and you could rest a cup of coffee on the throttle body cover it was so smooth(don't worry i didn't). now the only problem i still have is the throttle still sticking. i let the truck get up to temp and take off down the road. as i get up to speed everything is fine, smooth shifting no jerks. now when i slow to a crawl i have to move into neutral because the throttle stays open at about 2200-2400 for three seconds then drops to 1100-1400 for three seconds and then not until i come to a complete stop for three-four seconds it goes back to idle 500 - 600. but then it never does it goes again until i shut it off cool down and restart. i'm pretty sure i didn't do anything wrong or miss anything as this was happening before i changed what i changed. i just need to know where to start and what to isolate so i can pin point what WTF is going on. BTW thanks guys for showing an interest and with troubleshooting procedures its greatly appreciated.

alright intake manifold gasket and valve cover gaskets changed, so the mystery antifreeze leak is done and done. also changed two 3/16 vaccumm lines. no codes started right up a little rough at first but once up to temp it settled down and you could rest a cup of coffee on the throttle body cover it was so smooth(don't worry i didn't). now the only problem i still have is the throttle still sticking. i let the truck get up to temp and take off down the road. as i get up to speed everything is fine, smooth shifting no jerks. now when i slow to a crawl i have to move into neutral because the throttle stays open at about 2200-2400 for three seconds then drops to 1100-1400 for three seconds and then not until i come to a complete stop for three-four seconds it goes back to idle 500 - 600. but then it never does it goes again until i shut it off cool down and restart. i'm pretty sure i didn't do anything wrong or miss anything as this was happening before i changed what i changed. i just need to know where to start and what to isolate so i can pin point what WTF is going on. BTW thanks guys for showing an interest and with troubleshooting procedures its greatly appreciated.

You are welcome.

Clean throttle body buttery fly and iac valve.

Check for any codes and then clear VCM of any code and drive it and let it relearn and reset.

Then recheck for codes.

Post back how that goes.

alright intake manifold gasket and valve cover gaskets changed, so the mystery antifreeze leak is done and done. also changed two 3/16 vaccumm lines. no codes started right up a little rough at first but once up to temp it settled down and you could rest a cup of coffee on the throttle body cover it was so smooth(don't worry i didn't). now the only problem i still have is the throttle still sticking. i let the truck get up to temp and take off down the road. as i get up to speed everything is fine, smooth shifting no jerks. now when i slow to a crawl i have to move into neutral because the throttle stays open at about 2200-2400 for three seconds then drops to 1100-1400 for three seconds and then not until i come to a complete stop for three-four seconds it goes back to idle 500 - 600. but then it never does it goes again until i shut it off cool down and restart. i'm pretty sure i didn't do anything wrong or miss anything as this was happening before i changed what i changed. i just need to know where to start and what to isolate so i can pin point what WTF is going on. BTW thanks guys for showing an interest and with troubleshooting procedures its greatly appreciated.

Check BEHIND the throttle body butterfly for carbon deposits, like MT said. If you can't see, then CAREFULLY rub your finger inside and on the BACKSIDE of the butterfly. Black = carbon from EGR return. It can hang the butterfly, and the TPS won't go to 0 angle.

Your TPS also probably hasn't re-learned normal range yet. It will after half a dozen OPEN loop warmed up drive cycles.

I think you cannot clear codes from any module except PCM, without a scan tool. Those codes will go away after x number drive cycles, IF NO ERROR IS DETECTED / FIXED.

FYI - cold motor operation means NOTHING. That's called OPEN loop. The computer is NOT reading any functions that influence NORMAL warmed up operation. The circuit on any sensor is OPEN, and is not accepting feedback from any sensor, because the computer is trained to look for how the motor runs warmed up.

Fuel delivery is MAXED on OPEN loop, just to make sure the motor gets running.

alright throttle body butterfly carefully cleaned. it wasn't too dirty. the IAC on the other hand was filty. so after a few erands ran, the reving while idle issue is still there. coming to a crawl in neutral it revs, complete stop for three seconds back to idle, only now idle is higher at 700-800. still no codes. the heat now is acting up(not to throw ya guys off or off topic, maybe related?)it gets luke warm when started and engine comes up to temp, then blows cooler and cooler further into trip. if i put the fan on low setting(one) it builds up warmer but switch to two and it gets colder. don't if its relavent or not thought i should get all issues out.

Your TPS also probably hasn't re-learned normal range yet. It will after half a dozen OPEN loop warmed up drive cycles.

I think you cannot clear codes from any module except PCM, without a scan tool. Those codes will go away after x number drive cycles, IF NO ERROR IS DETECTED / FIXED.

FYI - cold motor operation means NOTHING. That's called OPEN loop. The computer is NOT reading any functions that influence NORMAL warmed up operation. The circuit on any sensor is OPEN, and is not accepting feedback from any sensor, because the computer is trained to look for how the motor runs warmed up.

Fuel delivery is MAXED on OPEN loop, just to make sure the motor gets running.


The TPS does not require relearn. It's plug and play.

The ONLY way to clear DTC's from the history file in the ECM is with a scan tool. Disconnecting the battery will turn the SES light off only if there are no hard faults present.

During open loop operation, the ECM uses MAP, MAF, TPS, IAT, and ECT to determine fuel mixture and ignition timing. The oxygen sensors are the only sensors that are ignored.

Fuel pressure is at maximum, (60psi to 66psi) only when manifold vacuum is zero: Key on engine off, and wide open throttle.

Heat and idle problem together could be a vacuum problem.
Check all vacuum and vacumn lines and hoses and vacumn cans real close and do not forget PCV valve and TB base gaskets and brake boster are under vacuum to.
Good Luck

thanks for taking the time to get back to the post. is there maybe a diagram somewhere on this forum to help me locate some or even all the vacuum lines that would most likely be associated with these issues. i have a haynes manual but that thing is not that great(or maybe im not that great at reading it). either way the manual i feel is better at explaining the suspension and other things non engine related. just my opinion. any and all suggestions are greatly appreciated.

thanks for taking the time to get back to the post. is there maybe a diagram somewhere on this forum to help me locate some or even all the vacuum lines that would most likely be associated with these issues. i have a haynes manual but that thing is not that great(or maybe im not that great at reading it). either way the manual i feel is better at explaining the suspension and other things non engine related. just my opinion. any and all suggestions are greatly appreciated.

look under the hood.

Some have vacuum diagram under there.
If not the Chev parts dealer may be able to print out one from vin no.
Factory repair manual or all data should have them in it.
http://www.alldata.com/products/diy/index.html
And sometimes AZ web site has some.

Or just take a good look at vacuum hoses and stuff on there and check for leakage.

so while getting my late night fix of candy, the SES light came on, while the issues were happening of course. scaner today said:

1 of 3-P0507
2 of 3-P0507
3 of 3-P1509

now as all these are related to IAC, which i just cleaned, is this a sign to replace or did i just not positioned it back into place right? i've gone 51 miles since cleaning IAC, about six or seven stops where i shut the engine off(going to the store etc.). as i turn the engine over in the morning, after it cranks, it idles at 1100 then drops to 700-800 as it warms up(takes about 6-7min to warm,don't know if thats relavant or not).but as i drive and get up to speed(30-40)and then slow down i have to push the clutch in and take it out of gear and the rpms go to 2400(in neutral)then drop to 1200 then if i don't stop and just coast or crawl in neautral then they jump up to 19-2100, and not until i come to a complete stop about 4 seconds later the rpm drop to 7-800.
BTW why the same code twice? thanks for the help

so while getting my late night fix of candy, the SES light came on, while the issues were happening of course. scaner today said:

1 of 3-P0507
2 of 3-P0507
3 of 3-P1509

now as all these are related to IAC, which i just cleaned, is this a sign to replace or did i just not positioned it back into place right? i've gone 51 miles since cleaning IAC, about six or seven stops where i shut the engine off(going to the store etc.). as i turn the engine over in the morning, after it cranks, it idles at 1100 then drops to 700-800 as it warms up(takes about 6-7min to warm,don't know if thats relavant or not).but as i drive and get up to speed(30-40)and then slow down i have to push the clutch in and take it out of gear and the rpms go to 2400(in neutral)then drop to 1200 then if i don't stop and just coast or crawl in neautral then they jump up to 19-2100, and not until i come to a complete stop about 4 seconds later the rpm drop to 7-800.
BTW why the same code twice? thanks for the help

As with any codes they do not say to replace but check/test it out.

The codes you have are telling you have a high idle which you already know.

Next step is test IAC valve or replace it.
But make suire all vacumn is good first.

man MT yer quick! yer also right i don't wnat to replace things just to replace them. i'm gonna spend some time makeing sure the vacuum is good and then test the IAC. how do you actually test the IAC?

man MT yer quick! yer also right i don't wnat to replace things just to replace them. i'm gonna spend some time makeing sure the vacuum is good and then test the IAC. how do you actually test the IAC?

They make special IAC valve testers also a good engine capable scanner has some test in it.
Also good scanner will give IAC valve position reading.

But for a one time test the testers or even having a repair shop test it are a lot more than what a New IAC valve would cost.

Good Luck and let us know how it goes.

so i was about to replace my IAC valve but noticed when i pulled it out that tha o-ring seal was torn. i have an o-ring seal kit for faucets and stuff so i just replaced it. started up er up it reved like it should then fell to 600 and stayed! i let it warm up for about ten min. took off home to have some beers( to nice of a weekend to mess with this). on the way not a single issue. rpms were fine, they would fall as the clutch was engaged and climb bcak up ONLY if i gave it gas. hope this is it, only been about 12 miles since reconnect and dtc cleared so we'll just to wait and see.

That sounds like a winner.
Thanks for posting back how it went.
Good Luck

this damn idleing issue is back. it is definetley not as bad as before. ive put about 200 miles and two gas fill ups since changing the o-ring. no code, yet. i wish i could say that when i drive or shift certain ways that it happens but it really seems like it just revs whenever it wants to. i did notice that this time i can hear engine rev when i'm in gear. so when i was going down a hill today i was going about 35 and in 4th at 13-1500rpm. i shifted into neutral and it went to 1700 and stayed until i got the bottom of the hill. and only after i came to a complete stop(4seconds) did it drop to idle,600-650. so i'm leaning towards the IAC not being bad since the idle is better, but where does that leave me...?TPS?Vacuum leak? whats a good way to check for those?