LT 4 idle and misfire.
04-13-2018, 04:19 PM
#21
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First thing I did was to check for vacuum leaks the old school way, starting fluid sprayed on vacuum lines. Saw no changes or Rpm rise. I’ll check again. When I cleaned MAFS I took some of that cleaner cleaned the throat of TB. Didn’t have any throttle body cleaner here and was afraid to spray that cleaner into engine. Will pick up some TB cleaner and use. Thanks for your ideas..!
04-24-2018, 05:23 PM
#22
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Continued surging
Cleaned MAFS and throttle body again. Looked more for vacuum leaks but none found. Changed PCV valve. Only code car has ever shown was P0154, O2 sensor 2nd bank , sensor 1. Has only shown up twice and cleared itself both times at highway speed. So I bought the correct O2 sensor, called a garage and took it in to have changed. Mechanic not as old as my hunting boots says 02 won’t cause that surge but he can find it. Anyway...Two hours later he’s says think it’s the intake but I need to take to Chevy place. Older mechanic looks it over and gets it to idle temporarily ok by messing with the opti vac hoses. Says leak is down there but will need to pull distribution to be able to get to hoses and bring it back and leave it so it can be cool. As you might guess this is gonna be costly. My question is can I get to and change the vent line and vac line on the opti without pulling water pump..?
Last edited by Capt. Ron; 04-24-2018 at 05:33 PM.
04-24-2018, 06:44 PM
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04-24-2018, 10:27 PM
#25
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Cleaned MAFS and throttle body again. Looked more for vacuum leaks but none found. Changed PCV valve. Only code car has ever shown was P0154, O2 sensor 2nd bank , sensor 1. Has only shown up twice and cleared itself both times at highway speed. So I bought the correct O2 sensor, called a garage and took it in to have changed. Mechanic not as old as my hunting boots says 02 won’t cause that surge but he can find it. Anyway...Two hours later he’s says think it’s the intake but I need to take to Chevy place. Older mechanic looks it over and gets it to idle temporarily ok by messing with the opti vac hoses. Says leak is down there but will need to pull distribution to be able to get to hoses and bring it back and leave it so it can be cool. As you might guess this is gonna be costly. My question is can I get to and change the vent line and vac line on the opti without pulling water pump..?
ETA: My original advice remains. I strongly recommend doing a datalog to figure out what the sensors are seeing and what the PCM is commanding. Again, the fact that it runs fine in open loop mode (when cold) tells us this probably isn't a constant mechanical problem like a vacuum leak. The Factory Service Manual for a 96 C4 will contain lots of troubleshooting decision trees for issues like this.
Last edited by MatthewMiller; 04-24-2018 at 10:33 PM.
04-24-2018, 10:58 PM
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Wait, I don't understand this at all. The vent hose to the opti from the intake just sucks in air. There's no restriction on it at all. So it's already an open "leak" by design. The hose could be full of holes or completely disconnected and it wouldn't change the amount of air entering the engine. All of this is to say that this isn't a source of a vacuum leak. A leak would be a place that doesn't normally suck air into the engine that's now sucking air in; whereas the opti vent hose always sucks unrestricted air into the engine. Something here isn't making sense.
ETA: My original advice remains. I strongly recommend doing a datalog to figure out what the sensors are seeing and what the PCM is commanding. Again, the fact that it runs fine in open loop mode (when cold) tells us this probably isn't a constant mechanical problem like a vacuum leak. The Factory Service Manual for a 96 C4 will contain lots of troubleshooting decision trees for issues like this.
ETA: My original advice remains. I strongly recommend doing a datalog to figure out what the sensors are seeing and what the PCM is commanding. Again, the fact that it runs fine in open loop mode (when cold) tells us this probably isn't a constant mechanical problem like a vacuum leak. The Factory Service Manual for a 96 C4 will contain lots of troubleshooting decision trees for issues like this.
04-24-2018, 11:02 PM
#27
Le Mans Master
That's correct, 95 was the first year. It pulls in fresh, filtered air from the intake snorkel, through the distributor, and then into the vacuum port on the intake. So that's just air being sucked into the engine all the time, no restriction.
07-15-2018, 05:45 PM
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Continuing problem...
In addition to what’s mentioned in the above posts...I now have replaced the IAC, the TPS, looked for more vacuum leaks...No change. Took it to Chevy place. They called said that they’re not sure what the cause is...Tested sensors, checked for leaks, nothing they did stoped the idle surge. They are recommending a new ECM but no guarantees....
07-15-2018, 09:48 PM
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07-15-2018, 10:16 PM
#31
Great thread! My 96 LT4 has a multipul misfires code as well. Like, if i am in 4th rolling at 30 mph, the car has a backfire hiccup thru the intake if i get on it, but then runs like gangbusters after. My idle is 1000 rpm.
Last edited by MikieG1971; 07-15-2018 at 10:17 PM.
07-16-2018, 11:22 AM
#32
Intermediate
My 94 auto runs normal in closed loop but when warmed up will surge in drive and the idle will fluctuate. Recently while experiencing the issue I disconnected the engine coolant temperature sensor on the water pump and the idle was normal. Plugged it back in and the symptoms returned. I have a new sensor to install once I have some free time and feel like wrenching in 100+ degree heat.
07-16-2018, 11:31 AM
#33
Le Mans Master
My 94 auto runs normal in closed loop but when warmed up will surge in drive and the idle will fluctuate. Recently while experiencing the issue I disconnected the engine coolant temperature sensor on the water pump and the idle was normal. Plugged it back in and the symptoms returned. I have a new sensor to install once I have some free time and feel like wrenching in 100+ degree heat.
07-16-2018, 12:17 PM
#35
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07-18-2018, 08:59 PM
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Injectors
[QUOTE=96GS#007;1596980198]Check the impedance on the injectors. They should be around 12 ohms. Injectors going bad drive the PCM crazy (via the o2 sensors) once it enters closed loop. Have seen this on several C4s and now that the LT4 is 22 years old, it’s bound to happen on some of them.[/QUOTE
All 8 Show 12 to 12.5.....??
All 8 Show 12 to 12.5.....??
Last edited by Capt. Ron; 07-18-2018 at 09:00 PM.
07-24-2018, 08:20 PM
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Back on my own.
Chevy place checked every sensor, unplugged vacuum hoses to check, removed throttle body cleaned with new gaskets. Corvette guy came to see...ordered new PCM. The one that came in could not be programmed so sent back and they just said come get car we don’t know...?? They have much better scanners and skill than me so back on my own...could it be PCM..? What sticks key in my mind is the idle only acts up when speedo says 0. 1 MPH and idles fine...Coukd PCM be bad...Car runs great, looks great. Still a slight miss in 6th if your still at 60 but smooths Out. I’m am a loss...Any good Corvette shops in SE GA or NE Fl you can recommend. Done all I know to do...
07-26-2018, 02:58 PM
#38
Racer
Do you have an obd II scanner that does live data? My car had the erratic idle when the engine coolant temperature sensor connector was on it's way out. It randomly jump way up in rpms for a few seconds and drop back down like it had a mind of its own. The idle jumps when the connector loses connection. At full resistance the computer reads this as -140F and tries adjust for it (really high idle). You will see it in the odb II scanner. This is the sensor that goes into the front of the engine. Once I saw the -140F on the scanner when the idle went high, I proved it was the connector by cutting it off, stripping the wires and using alligator clips to by pass and it ran perfect.
07-26-2018, 05:01 PM
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Scanner
I do not have that type of scanner but if someone can recommend one I’ll look into it...Tired of this problem. Mechanic at Chevy place had seen a post on here similar and he said he unplugged this sensor and made no difference. Is this the sensor that goes to the analog gauge..it seems to run hotter than the digital one. Thank you for your help.
07-26-2018, 05:52 PM
#40
Le Mans Master
Unplugging the sensor would cause it to see infinite resistance though, which tells the computer the engine is at -140F. The mechanic would have had to jump across the two terminals of the connect, like haxx did (except maybe you can do it without cutting off the connector by using a paper clip or something). Then the computer would think the engine is really warm, and if that makes it run well then you know it's the temp sensor.