Rev Drop on Hard Acceleration
 

Ok so here is the problem. When i am cruising along at let say 60km/hr in 3rd gear and go to step on the gas the revs will climb normally to about 3500rpm and then drop back about 500rpm and then climb again all the way to redline with no hesitsation. What is causing this lag in the rpm's upon inital acceleration? The same thing also happens in 4th and 5th gear when a stomp on the gas while crusing along around 2000-2500 rpm.

I hope that someone on here might have the solution since he dealer is not doing so well at locating the problem.

The dealer has replaced the cam position sensor becuase it was showing that code when the hooked it up to reader, but that did not solve the problem.

Any ideas fellas?

only thing i can think of is either a leak or something failing on the fuel system.

Possible clutch slipping?

clutch slipping would be the opposite, the rev's would hang low and then jump up. from what he's describing is its accelerating normally and then the rpm's drop and then resume after a few seconds.

Guess I was confused as to what "step on the gas" means in his mind.

Edit: oops I read it wrong, thought you said it jumps to 5000 rpm. nvm...

"outamymind" you are right.

what i ment by step on the gas is a hard acceleration within the gear. It only seems to happen at this point. If i baby the gas and let the revs climb slowly within the gear it doesnt seem to have the rev drop.

Thanks for the quick replies guys, i am new to the site and think its great!!

Lmao so clutch slippage is when the rpm stays at a rpm of say 2500 then jumps to 500more rpm..... Ummmmmmmmm

...... Clutch slippage is when the friction disc, pressure plate, and the flywheel are not joinin together causing slippage which in return is this..........
When u get a load on the car is say 3,4,5 gear and the rpms rev high for some time and comes down to a lower rpm wether it is 1-30 seconds its still clutch slippage unless its tire spin..

see i was always going by clutch slippage as what you described but rpm's increasing without the speed of the vehicle increasing and then dropping quickly and then the vehicle picks up speed.

from the OP's description is he's increasing in speed and rpm's and then the rpm's drop. i could be just misreading what the OP has typed but to me from how i see it, this isn't clutch slipping

Chevycobaltss3--if it is clutch slippage would the revs not shoot to the moon while its slipping due to the fact that i am still hard on the gas when the revs are dropping??

Depends on how bad thr clutch is out..

what do you mean by "depends on how bad the clutch is out?" do you mean its out alot or just starting to go? I do not notice it slipping on intial shift.

Any other ideas as well to what it could be? im trying to get some more info to go to the dealer with.

P.S thanks a lot for the help.

i'm still leaning towards something cutting out while your stepping on it then a slipping clutch but thats just me, subbed for results.

Outtamymind- any ideas as to what by be cutting out?

without having some data logs could be almost anything, my best guess is the fuel pump. but only real way to test that is to put a pressure gauge on test port or a tech 2 or HPt to monitor fuel pressure when it does it.

Meaning how much the friction disc is glazed and ect....... I have multiple vids of cars ive tuned that showed signs of clutch slippage and some ranged from 500-1000rpm and others 2000-4000rpm.... If u put s load on the car and thr clutch cant take it.it will slip till it csn hold it..... If he was having a fuel cut or say a misfire from hell he would hear and feel it and more then likely throw a code..

i guess the only thing that i dont understand is that if i am puttin load on my car (aka hard acceleration) and the clutch cant take it and is slipping, why would the revs drop? i would think they would rise, since i am still on the gas when it drops.

My first thought was spark blow out.... MAYBE....
But my next thought does not even apply to our cars. But might in a sence.

I remember in old carburated cars, they had a pump on the carburator, so that
when you suddenly give it alot of gas, it pumps in extra fuel, to compensate for
the sudden gulp of air without fuel.

But now with fuel injection, the accelerator pump from the carburaters is no longer relevent.
But maybe its posible something SIMILAR is happening, such as,

Some how when you stomp on the gas,
you get alot of extra airflow,
and your PCM/FUeling system is Sluggish in increaseing the fuel delevery.

So I would tend to lean and agree with the person who said something about fuel system.
Do you have a wideband? When you stomp is there a moment when it goes lean, as it dies down?
Also maybe something in the Throttle Body position, or the Mas Airflow Sensor. But youd think youd get a code of one of those was wrong.


EDIT: Just remembered my fuel injected Dodge Shadow would do the same thing, but not really bad. Then one day it got REALLY bad (a week before it was traded in for my Cobalt) and it was throwing a MAP sensor code, and an O2 Sensor code.

off topic but thats too weird, i traded in a 91 dodge shadow for my cobalt too lol.

back on topic, only real way to know if its fuel related or not is to hook up a pressure gauge and monitor fuel pressure, or get a wideband and check to see if it leans out. HPt should be able to monitor this as well and give you fairly accurate readings on whats going on with the system when it does it.

Just came to me: Id put my money on a fuel filter!
If the filter is clogged, when your driving normaly, the fuel pump keeps the system pressurised.
When you baby the gas, the restriction of the filter isnt enough to cause problems.
When you gun it... the sudden demand causes a temporary loss of pressure and fuel downstream of the filter, and the pump then works harder and ups the pressure to compensate, resulting in a lag of fuel delivery, then when the lag is over, your car takes off like normal.
I could be wrong though.

Mine was a '93, really good car, Id have kept it and fixed it up as a project car if Id have had the garage space.

I agree, put on a fuel pressure gague.
If you want get an interceptor and a wideband kit and the fuel pressure kit,
and hook them all up. :)
Youll need a way to test your fuel pressure while under varying load. So unless you hook up a way to monitor fuel pressure whiel you drive, youll have to use a fuel pressure tester while having the car dynoed. I doubt just doing a neutral rev in the drivway will give you reliable results.
BTW: Mine runes normaly between 60 - 65PSI, (though sometimes a few PSI less then 60)
I have aeroforces fuel pressure kit on mine.

Also: A Sticking pressure releif valve on the fuel pump could cause this. Again, a fuel system pressure test is required.

ya i've scene some people get a long enough hose and run the pressure gauge in front of the hood and drive it (not recommended to do this lol)

and ya the shadow was a good car, she handled every beating i gave her with ease. but it started having tranny issues and would die when i went to shift from time to time, so it was time to trade her in.

Look at ure post above or ure issue you said.. Wut ure saying right now does not match wut u put on top.. If ure revs go high on a hard accel and come back down after a few second then look at clutch.. If it drops then comes up then id say u have a failing sesnor, fueling issue, something needs diagnosed which means more depth about wuts.going on.

OP lets clarify something. when you go hard on it and the rpm's go up does your car pick up speed?

Chevycobaltss3----

I guess it is just the way i am wording it.

upon hard acceleration the cars rpm will rise normally till it reaches appox 3500rpm, at that point the rpms will then drop about 500/rpm and then continue to climb all the way to redline if i so choose with no further drop in rpms.

I dont believe i ever stated the the revs shot up then came back down. i was saying that the revs climb normally and then drop off about 500 at the 3500/rpm point. In the last reply i wrote i was commenting on what you said. (playing devils advacat, so to speak).

yes the car picks up speed when i go hard on it.

Everything is normal on acceleration until i hit the 3500/rpm mark. Thats when it drops off 500rpm and then continually climbs all the way to redline (if i were to let it do so).