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'89 Cavalier 2.8L Z24 won't accelerate over 25mph, car just jerks.

06-03-2012, 03:35 PM
No trouble codes from the CPU come up, engine fires up fine, idles good, drives real smooth up until ~25 mph. Then the engine just doesn't respond to the gas pedal, engine does not shut off. Once the car hits a certain speed, it just wont accelerate anymore, once the vehicle slows down a bit it will let me accelerate again until it hits a certain speed again and the engine becomes unresponsive to the throttle again. I've changed the fuel filter, checked the fuel pressure, checked the EGR valve, changed the TPS sensor, but nothing. Please Help...

Tech II
06-03-2012, 04:18 PM
Check exhaust for blockage(at cat?)?

06-03-2012, 07:02 PM
Check exhaust for blockage(at cat?)?

Took the exhaust off from the exhaust manifold and back, made the car loud but still kept jerking at 25mph... rules out exhaust blockage.

Tech II
06-04-2012, 08:49 AM
Secondary ignition?

06-04-2012, 09:37 PM
Secondary ignition?
I'm not too familiar with the ignition system, but this vehicle use coil packs for the ignition system, two cylinders per coil pack, total of 3. Does secondary ignition still apply to this type of set up? If so, could u please elaborate. Thanks.

Tech II
06-04-2012, 09:47 PM
Secondary ignition is just a generic term for the output voltage to the plugs..........what you will need to do, remove one wire at a time from a plug, and attach a spark tester......then start the car, and check to see if you have a healthy spark at the tester......do ALL SIX WIRES.

Be careful......you don't want any sparks near the battery......hydrogen gas is explosive.....

06-06-2012, 12:14 AM
I checked the spark on all 6 wires, and all 6 had a healthy spark. So I decided to check the gap on the plugs, and to my surprise one of the plugs was completely destroyed. On top of that there was a small amount of fuel on the plugs. I went ahead and replaced the plugs and decided to check the fuel pressure, pressure is at what it should be and the fuel pressure regulator is working as it should. I even disconnected the vacuum to the regulator and fuel pressure went up as it should. The car is also giving out a gassy smell, as if its running rich. I'm not sure were to go from here as far as what to check next...:banghead:

Tech II
06-06-2012, 11:31 AM
While you had the plugs out, should have checked compression....

06-07-2012, 03:02 PM
I checked the compression on all cylinders, and all were healthy (120-145 psi). I can definitely tell the car is running rich from the exhaust fumes. And another thing i noticed was the spark plug cables were not set up properly, I set them up according to the Haynes repair manual and when I started up the car I could tell it was miss firing. Now I am just confused, I double checked the wires and they were as they should be according to the book.

Tech II
06-07-2012, 05:26 PM
There should be an emission label showing the coil/wire location, and they should be numbered.....good compression and spark? Then we have a mixture problem, check MAP, MAF, O2 sensor....could even be injectors....

06-09-2012, 05:20 PM
Okay, update...
Oxygen sensor was replaced last week, this vehicle is not equiped with a MAF sensor and the MAP sensor has been checked and found to be working. I have yet to double check the injectors but from I was told they were replaced a year ago. One thing I have noticed is I took the car for a spin after checking the MAP sensor and went on a downhill so that the vehicle can go at the speed when it does not let me accelerate, I held the clutch in and let the car cruise down the hill and stepped on the gas, and no response. The vehicle had no load, just cruising on neutral and it still would no let me accelerate.
One thing I failed to mention from the start is the speedometer does not work. Reason I never mentioned this was I just thought It was an instrument cluster malfunction, but now I am starting to think I have a faulty VSS, no trouble code has come up for faulty VSS however. Now my question is, would a faulty VSS cause this type of malfunction? The VSS equipped in this vehicle is a magnetic type with no speedometer cable, all electronic.

Edit: spark plug wires were corrected, two of the cables were switched, vehicle is running smooth again.

Tech II
06-09-2012, 09:13 PM
Clutch slipping? RPM's going up with no increase in speed?

06-10-2012, 01:36 AM
Clutch does not slip at all, RPM does not go up as well. Engine does not rev up whats so ever, just idles at ~900 RPM. It's as if I'm not even pressing on the gas pedal even though I am. It's as if once I hit a certain speed, something tells the car not to accelerate any more even if I press on the gas pedal.

08-01-2012, 09:29 AM
Sounds like a CPU problem to me.

The only other thing I would be checking out is what the factory specs are for compression. Those numbers seem low to me and I would be wondering if it isnt possible for your timing chain to have skipped ahead a tooth. It would be enough compression to run but not efficiently. Is a bit of a long shot but 120psi seems like a low number to me. If they are all down that amount then I would be suspecting chain to have skipped. I suspect original numbers should be closer to 180psi

Tech II
08-01-2012, 10:51 AM
Minimum is 100psi.......180 is too high.....

08-01-2012, 11:05 AM
Yeah I figured it was a long shot. I still think it sounds CPU although my initial feeling was fuel pressure until I read the post where the fuel pressure tested fine (including dead heading the pump by shutting the vaccuum off to the regulator). Only thing I note is that the readings were not given. If someone was going on the TBI #'s (15-20psi) instead of the MPFI #'s (35-40 regular running and upwards of 60 under full pressure) that may be where the error lies. Wonder what the readings obtained were.

08-03-2012, 05:50 PM
Will need to hook it up to a scanner and monitor what's going on.
I would suspect the TPS (throttle position sensor).

What could be causing this...
1) insufficient fuel pressure (pump not keeping up, bad regulator)
2) timing not advancing properly (bad ignition control module)
3) Computer thinks everything ok
- bad prom
- bad sensors (like TPS) flatline and lie to computer
- maybe a physical interference on throttle

If timing chain skipped a tooth it should be idling like a dog.

08-03-2012, 06:51 PM
the compression could all be 50 psi it would still rev up and accelerate you don`t have a compression problem. I would get a scantool & look at injector pulse with sounds like your not getting fuel delivery to the engine. should be TBI if so you can look in the TB and rev it up under the hood and see your fuel spray just dumping everywhere at WOT. IF you can`t then you have got injector or PCM problem. signed, always stuck in diagnostics!:banghead:

Joe W
08-04-2012, 12:45 AM
This does seem to be a possible VSS issue, although I must question why a code has not been set, the same reguarding the CPU, even through failure they throw a code. Have you tested the VSS?

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