Engine running rough
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
Engine running rough
Hi folks,
I'm going to share as much detail as possible about the issue in case someone on the forum has had a similar experience and can provide some guidance.
I have a '96 LT4 and a couple of days ago as I got home and parked in the garage, I started feeling an unusually strong vibration from the engine. The vibration is rhythmic with the RPM, it's not sporadic, and it's not a hum, buzz, or drone.
I drove it around the block the next day, still had the same excessive vibration at low RPM, and hesitated and sputtered in acceleration, with the power surging and dropping sporadically.
When cold, the idle is rough and the RPM keeps fluctuating ever so slightly, almost as if the engine keeps trying to correct itself but falls down again.
And strangely, there are NO CODES (yet)!
UPDATE: CODE P0300
I'm going to go through a list of potential culprits:
1. The MAP sensor: I cleaned up and put it back in, but no luck (also broke the connector into pieces so will be replacing that)
2. A vacuum leak: I'll test the vacuum lines to see if there's any leaks anywhere. Visually, none of the rubber looks disastrously dry or breaking.
3. Fuel injectors: Been wanting to replace the fuel injectors for a while, and this seems like a good opportunity to get it done.
4. Alternator: Maybe unlikely that the alternator is the cause of the issue, but I have always had a very slight fluctuation in the instrument cluster's back light. I just always chalked it up to it being an old car!
5. Optispark: I know that the 1995/1996 cars aren't supposed to have optispark issues (or not as much) but I'm guessing if all else fails, that might be worth a look, too.
Fuel filter and spark plugs were replaced about 3k miles ago.
If anyone has had experience with a similar issue, or can help provide guidance in general before I dive in, it would be highly appreciated.
Thank y'all in advance!
I'm going to share as much detail as possible about the issue in case someone on the forum has had a similar experience and can provide some guidance.
I have a '96 LT4 and a couple of days ago as I got home and parked in the garage, I started feeling an unusually strong vibration from the engine. The vibration is rhythmic with the RPM, it's not sporadic, and it's not a hum, buzz, or drone.
I drove it around the block the next day, still had the same excessive vibration at low RPM, and hesitated and sputtered in acceleration, with the power surging and dropping sporadically.
When cold, the idle is rough and the RPM keeps fluctuating ever so slightly, almost as if the engine keeps trying to correct itself but falls down again.
And strangely, there are NO CODES (yet)!
UPDATE: CODE P0300
I'm going to go through a list of potential culprits:
1. The MAP sensor: I cleaned up and put it back in, but no luck (also broke the connector into pieces so will be replacing that)
2. A vacuum leak: I'll test the vacuum lines to see if there's any leaks anywhere. Visually, none of the rubber looks disastrously dry or breaking.
3. Fuel injectors: Been wanting to replace the fuel injectors for a while, and this seems like a good opportunity to get it done.
4. Alternator: Maybe unlikely that the alternator is the cause of the issue, but I have always had a very slight fluctuation in the instrument cluster's back light. I just always chalked it up to it being an old car!
5. Optispark: I know that the 1995/1996 cars aren't supposed to have optispark issues (or not as much) but I'm guessing if all else fails, that might be worth a look, too.
Fuel filter and spark plugs were replaced about 3k miles ago.
If anyone has had experience with a similar issue, or can help provide guidance in general before I dive in, it would be highly appreciated.
Thank y'all in advance!
Last edited by Thepersian; Mar 19, 2024 at 12:08 AM.
Melting Slicks
Joined: May 2003
Posts: 2,076
Likes: 281
From: Dyersburg TN
have you checked the fuel pressure. I had a pump fail but not all at once. It would loose pressure and regain it randomly. Then engine behaved very much like you describe. I did not catch it at first because I was used to pump failures that stayed failed not wandering in and out of spec. I would also suggest checking the connectors on the ignition parts to ensure no bad connections. Good luck in your search.
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
have you checked the fuel pressure. I had a pump fail but not all at once. It would loose pressure and regain it randomly. Then engine behaved very much like you describe. I did not catch it at first because I was used to pump failures that stayed failed not wandering in and out of spec. I would also suggest checking the connectors on the ignition parts to ensure no bad connections. Good luck in your search.
When looking at the connections, any particular signs of wear I should look out for?
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
have you checked the fuel pressure. I had a pump fail but not all at once. It would loose pressure and regain it randomly. Then engine behaved very much like you describe. I did not catch it at first because I was used to pump failures that stayed failed not wandering in and out of spec. I would also suggest checking the connectors on the ignition parts to ensure no bad connections. Good luck in your search.
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
Update:
Replaced the fuel pump, and while the car runs slightly better with the new pump and has better throttle response, it's still running rough. It idles with excessive vibration, has loss and surges of power in mild acceleration, and the exhaust fumes have a burnt smell to them. I'm about at the limit of my knowledge with this issue. Might replace the fuel injectors as well, since I already bought a set. But otherwise, I think I might have to take it to a shop for a (hopefully) more accurate diagnosis.
Replaced the fuel pump, and while the car runs slightly better with the new pump and has better throttle response, it's still running rough. It idles with excessive vibration, has loss and surges of power in mild acceleration, and the exhaust fumes have a burnt smell to them. I'm about at the limit of my knowledge with this issue. Might replace the fuel injectors as well, since I already bought a set. But otherwise, I think I might have to take it to a shop for a (hopefully) more accurate diagnosis.
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
Does it have issues the moment you fire it up? Or does it run fine when cold and as it warms up the issues develop?
the reason I’m asking is that i recently had some issues, but it ran fine cold(open loop) and when it warmed up, it switched to closed loop which is when I had issues. This cued me into that it was a sensor fault.
have you double checked spark plug wiring? Ensured that no wire is incidentally against the exhaust manifold?
your description sounds like it actually has a miss. You could pull plugs and see if you could find the offending cylinder(s)
the reason I’m asking is that i recently had some issues, but it ran fine cold(open loop) and when it warmed up, it switched to closed loop which is when I had issues. This cued me into that it was a sensor fault.
have you double checked spark plug wiring? Ensured that no wire is incidentally against the exhaust manifold?
your description sounds like it actually has a miss. You could pull plugs and see if you could find the offending cylinder(s)
Last edited by randallsteel; Mar 31, 2024 at 08:40 AM.
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Thread Starter
Intermediate
Joined: Jul 2018
Posts: 33
Likes: 27
From: los angeles
Does it have issues the moment you fire it up? Or does it run fine when cold and as it warms up the issues develop?
the reason I’m asking is that i recently had some issues, but it ran fine cold(open loop) and when it warmed up, it switched to closed loop which is when I had issues. This cued me into that it was a sensor fault.
have you double checked spark plug wiring? Ensured that no wire is incidentally against the exhaust manifold?
your description sounds like it actually has a miss. You could pull plugs and see if you could find the offending cylinder(s)
the reason I’m asking is that i recently had some issues, but it ran fine cold(open loop) and when it warmed up, it switched to closed loop which is when I had issues. This cued me into that it was a sensor fault.
have you double checked spark plug wiring? Ensured that no wire is incidentally against the exhaust manifold?
your description sounds like it actually has a miss. You could pull plugs and see if you could find the offending cylinder(s)