I've spent hours looking for answers but I didn't find anything that really matched my symptoms and made sense. Little help?

1996 Cavalier LS, 2.4L, 5-speed manual, 75k miles.

It's mostly original parts but the MAP sensor vacuum tube, fuel filter, water pump, timing chane, & tensioner have been replaced.

Idles fine. Generally runs well. But when accelerating at lower RPMs (say 2k or below) it hesitates and stutters. It accelerates and cruises fine at higher RPMs. No backfire. No CEL. No codes.

The problem appeared a little over a year ago, at which time I replaced the fuel filter & map sensor vacuum tube. The problem is now more pronounced, and seems to extend up to higher RPMs (say 2.5k or so).

Any suggestions? If you have a suggestion, would you please explain why it only affects the car when accelerating at lower RPMs?

Thanks in advance for any help.

I would either clean or replace the IAC sensor, and look at the TPs sensor. Also, GM did release a few revised PCM calibrations that helped this a lot.

Does it do it only when cold? Not up to operating temperature?

Aged LD9 motors "2.4l" tend to do this when they get older. Normally a good tune up... help / fixes / solves the issue. Sometimes you can have cracks in the white housing the holds the ignition coils, but normally you will have misfire codes if this is cracked, and messing with everything.

My money is on the IAC sensor probably.

There is a VERY slight chance that the front oxygen sensor is on its way out also.... especially if it is a bosch O2 sensor.








PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

Thanik you!!!

I'll check for certain, but I think the hesitation problem lessens (maybe almost disappears) when the engine is hot. (If it helps, I live in CA so it's ~ 70 degrees out when I'm driving.) Would that lead you to a different conclusion?

My wife and I both have 1996 2.4L Cavs so this weekend I guess I'll try to find the problem by swapping her IAC sensor. If that doesn't do it, I'll swap the TP sensor. If that doesn't do it, I'll swap the IAC. Then maybe the O2 sensor. And if none of that works, I'll take it to a shop.

Thanks again!!

I had the same issue on my 2.4. I replace everything I could find from the plugs and boots to O2 and IAC sensors - $500 later it still hesitated below 2k rpms

- I only recently fixed the issue - I replaced the ignition system by doing a coil swap from a neon - now it purrs like a kitten and haven't had any issues in the last 6k miles now
-Total swap costed my about 80 bucks - wish i would have started with that :/

my 99 Z did something similar to this. try unhooking the hose to the FPR as its running and see if gas spits out. if so, there you go. if not, it was worth a shot.


"If I'm not back in 5 minutes...just wait longer"

Thanks so much to everyone. I really appreciate the help. Here's an update...

1) Per MD Enforcer's question above, I confirmed that the problem does not improve when the car is hot.
2) Per MD Enforcer and Tahua's suggestions above (since my wife & I both have 1996 2.4L Cavs) I swapped with my wife's coils (which were replaced about 3 years ago), IAC sensor, MAP sensor, & TP sensor. That didn't solve the problem on my car nor create a problem on her car.
3) I also pulled my spark plugs and they all looked good (for 75k miles).

Per Killer Cavi's suggestion, next I'll look into the Fuel Pressure Regulator.

Let us know.




PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

SOLVED!!!
Eventually the hesitation got worse until my car just started misfiring and running very roughly. I limped into a trusted repair shop. The mechanic listened for about 3 seconds and told me to go home and replace the ignition housing and coils. I limped home and ordered them online, standard shippping. Two days later they arived and I installed them but the engine sounded a little rough, so I went down to my local O'Reilly's and got 4 spark plugs and 4 spark plug boots, which I installed. Now the car runs purrrrrfectly. No hesitation. She's like brand new. I'm very happy.

Since my wife also has a 1996 Cavalier 2.4L and since I'm a cheap bastard, I wasted a lot of time swapping ignition parts with my wife's car to determine whether I only needed to replace the ignition housing or just one of the coils, but it wasn't worth the time-- especially since it didn't really run perfectly and I could not isolate the problem to one part. My advice to anyone having this type of problem is (for a lousy $78) to replace everything (housing, coils, boots & spark plugs).

Thanks to MD Enforcer, Tahua Ravenous, and Killer Cavi for the help!

Parts & cost:
$23 for a new Prozone ignition coil on ebay (which included new spring contacts, jelly, etc.). Free shipping.
$27 for 2 new Herlux ignition coils on eBay. Free shipping.
$12 for 4 new Autolite platinum spark plugs.
$16 for 4 new Omnispark coil-on-plug insulator boots.
-------------
$78 Total.

I should add that this is really an easy job-- maybe 30 minutes. Here's how.

1) Remove the 4 (13mm) bolts from the ignition coil housing cover. (It's the metal plate on top of the engine that says, "2.4L TWIN CAM".
2) Remove the electrical connector from the driver's side of the housing cover by usng a screwdriver to remove the green locking pin and then pressing down on the black locking tab while pulling off the connector.
3) There are two aluminum fuel lines that run over top of the electrical connector that are held in place by a clip with a 10mm hex bolt. If you remove that bolt it will be easier to remove the housing.
4) There are two black lines that run across the top of the housing cover. If you remove them fom the clip it will be easier to remove the housing.
5) Wiggle up the ignition coil housing cover to remove the assembly. Make sure that all 4 black coil-on-plug insulator boots come with it or pull any remaining boots off by hand.
6) Place the ignition coil housing upside down on a workbench and remove the 4 black connector boots.
7) Remove the 4 small hex bolts connecting the white ignition coil housing to the metal housing cover.
8) Carefully remove the white housing and use a small flathead screwdriver to remove the connector inside. The two black coils may fall out of the housing.
9) Remove the wiring harness connectors from each coil using a small flathead screwdriver to pry the top & bottom tabs while pulling off the connector.
10) Put the old harness on the new coils and reassemble the coils into the housing (hopefully using new connector springs and silicone jelly).
11) Put the new black coil-on-plug insulator boots onto the housing. It's ready to reinstall. But first...
12) Change the spark plugs.
A) Use compressed air to blow any dirt out of the spark plug socket area so that it doesn't fall into the cylinder.
B) Use a spark plug socket on an extension on a socket wrench to remove the spark plugs.
C) If you didn't buy pre-gapped plugs, then gap the new spark plugs to 0.6mm using a spark plug gapper (~$1.50 at an auto parts store).
D) Carefully, by hand on the scoket/extension, screw-in the new spark plugs and then tighten with a socket wrench (to 13-pound-feet of tork).
12) Carefully reinsert the housing cover so that the black boots align over the spark plugs and press down snugly into place.
13) Reverse steps 4, 3, 2, & 1 above.

Thanks for the feedback.




PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

Two more things in case it helps anyone:

1) In step #7 above, I used a 7/32" socket. (I expected it to be metric but my metric sockets didn't fit.)

2) Yesterday I did the repair job and posted above. Today I was debating whether to throw out the old white ignition coil housing, because it looks fine. I used a volt-ohm meter (in resistance & continuity-check mode) to see if there was a good connection from the old contact springs (inside the ignition coil housing) to the contacts on the outside of the ignition coil housing. All four contacts tested perfectly (like almost zero ohms) and gave a smooth beep on my continuity checker. So I'm actually kinda doubtful that the problem was with the ignition coil housing, but I'm too lazy to try to swap out the old housing to confirm. Maybe I could have saved $23 by just replacing the coils, boots & spark-plugs. I'm not sure if using a volt-ohm meter (continuity checker) is a valid way to test the integrity of the ignition coil housing, but it seems like it should be.

Just out of curiosity I swapped my original white ignition coil housing onto the new coils, boots, & plugs, but it once again caused misfires. So I am confident that the original housing failed.

According to THIS WEBSITE:
1) Sometimes you can, and sometimes you can't see that cracks in the housing.
2) There is no way to test the housing.
3) When the housing fails, it causes a (high-voltage) arc to ground. (This may explain why a low-voltage volt-ohm meter can't detect a break in continuity.)
4) The arcing can damage the coils over time. (This may explain why it's recommended to replace the housing and coils.)

If you have a similar problem, I'd recommend that you replace the housing, coils, boots, & plugs ASAP, before the problem worsens as mine did, because after a while the engine will misfire, which will send unburned fuel into the exhaust system and cause the catalytic converter to quickly fail (as mine did). If you live in a state that requires smog-testing and your catalytic converter fails, then you will fail the smog test (due largely to high NOX if I remember correctly) and you'll have to replace your catalytic converter, which is about a $400 job requiring the old cat to be cut out and the new one welded in-- though most of the cost is the part, not the labor.