Hi all!

Ok I'll give a little background to my situation first, I have a 1988 Camaro 305 TBI (LO3, vin E) with manual 5 speed transmission.

I have had the car for just over 2 years and is stock apart from a decat exhaust, 160 degree thermostat and an aftermarket accel coil, there is 75,000 miles on the clock 8,000 of which I put on there. The car is my daily driver.

The problem started about 3 months ago and is now getting worse, heres what happens:

I start the car up from cold in the morning and drive to work, just before the engine reaches its normal peak operating temperature the car starts to jerk quite badly, this happens on a flat straight road at 45mph in 4th gear (2000rpm), pressing the gas pedal further makes the problem far worse, in fact the car won't even accelerate while this is happening. I just have to bear with it until it clears.
I should also point out that this jerking can happen at any speed and at any temperature (but very rarely) but mostly when its in warm up. I notice the rev counter also bounces when this happens.

My second problem is while accelerating, occasionally the power/acceleration drops off momentarily and the car feels all round slower than usual.

Are these two problems related? what is causing them? I am hoping that maybe its just the spark plugs that need changing but would this cause the severe jerking? I did a search for this jerking problem but it always turns up threads about this problem with automatic transmission cars which turn out to be converters etc.

I'd appreciate any help!

Scott

 

make sure your tune up is good, scan it for codes, and i'd check filters. make sure the emission ducting from the manifold to air cleaner is intack and the valve in the air cleaner is working

 

When you say Valve in the air cleaner, do you mean the heat stove valve?

 

To answer your question...if the spark plugs are bad enough to cause the engine to misfire, that will definitely cause jerking. Check your ignition wires also...it's possible one could be grounding out or could be cut, etc. I'd say that's more likely to be the case than a bad spark plug or two since the problem comes and goes. Sounds like a cylinder or two might not be getting good spark.

 

I can't seem to find any damaged/disconnected vacuum/emmision lines and the spark plug wires seem fine. There is no arcing to be seen anywhere. I have no trouble codes either.

I am confused as to why this problem occurs mainly at warmup after the engine reaches 50 degrees C.
Is this when the engine swithes into open loop and starts to rely on the sensors?

The rev counter bounces slightly when the car is jerking although the cars speed remains constant, is this an indication of spark events fluctuation in frequency? maybe if the coil was going bad?

Would running rich cause such bad jerking? would spark knock cause jerking?

Sorry for all the questions, this problem seems to have so many possible solutions

 

I'm afraid I don't know how to translate temperatures here...50 Celcius would equal about what in Farenheit?

Now the problem occurs mostly DURING warmup, but it goes away AFTER the car is warmed up? Not sure if any of the emissions components could be causing you problems....maybe the EGR, I'm really not sure...

But what I can say is this definitely sounds like you're getting spark dropped. Have you checked your spark plugs? If they look pretty rich then that could be an indicator you're either getting sh*t for spark, or...you're running rich obviously.

No, rich won't cause jerking like that. My car was running pig rich the other day...it was about 50 degrees out and it was so rich that it was leaving a trail of smoke behind me on the interstate for a couple hundred feet. Didn't jerk or act strange at all...just stunk up bad and burned your eyes if you put your head under the rear bumper.

Spark knock (as in detonation?)....I suppose that could cause jerking. Why would you think you might have spark knock? Do you hear any pinging or ticking, especially during acceleration in a high gear?

 

When I say 50 degrees Celcius I mean around 130-140 Farenheit (I'm running a 160 degree thermostat) so literally just before peak normal running temperature. Is this the closed loop switch on temperature? I don't know if that has a part to play in all this.

I would'nt say the problem goes away completely after warmup, but its definately at its worst when at the temperature mentioned above.

I have recently replaced my EGR valve about a year ago due to the engine cutting out after deceleration, that cured that problem!

When I asked if spark knock/detonation would cause the jerking I am merely trying to imagine the conditions inside the cylinders that would cause such a problem. I don't have any pinging sounds but I did wonder if detonation would cause this violent jerking. I did think that if it was rich, it would not cause this problem as you say but would instead just cause power loss and bad mileage.

It has to be either spark or fuel that needs my attention, if its fuel, well it could be anything from fuel filter to faulty sensors.

My next move should be to pull the plugs I guess But then why would the spark plugs be affected by such a narrow range of engine temperature and be fine when cold or hot but not warm?

 

That's a tricky one...I'm getting brain freeze here....

I wouldn't expect a problem like violent jerking to be linked with engine temperature... Hmm...the O2 shouldn't have any part with this problem....you said you replaced the EGR......I doubt the PCV would cause this. Seriously, the only available option that sounds viable to me is spark drop to certain cylinders.

Check all your plugs and spark plug wires, and check your ignition coil output. Check your timing also if you haven't already.

The not wanting to accelerate and jerking worse with more gas definitely sounds like a strong misfire.

The very next time it starts jerking and loses powerly badly like that...try popping the car into neutral and revving it. See if it revs normally or if it does any strange things like the engine jerks or bucks, backfires, etc. If it acts up in gear but acts fine in Neutral, that's surefire ignition.

 

CamaroUK,

You may have your problem fixed by now, but here's my experience -

I have the same engine in my 91 Firebird Formula 305 tbi. I have also had the same symptons you have, on two different occations. The symptons are really weird, but when I checked my spark plugs I saw that they were really carbon fouled (dull black powder all over the insulator, center electrode and side electrode). After I changed plugs it ran like new (both times). I couldn't believe that fouled plugs would cause the symptons that I had, but stuff happens.

Good luck,
Don

 

Sounds like you were running pretty rich...

And yeah, you'd be amazed how picky the engine is when plugs are concerned. Even changing from old non-fouled plugs to new ones can be felt in the seat of your pants usually.