Chevrolet Lacetti 1.6 80kW surges at constant speed low throttle.

I have this problem now for almost a year. It's alot better now than it was, but still the problem remains. I also have negative fuel trims all the time, rich condition. About -13 percent.

I've checked for vacuum leaks with smoke machine, solved them at the intake manifold gaskets, changed injector orings, changed throttle body, replaced pcv valve, cam sensor, ignition wiring, Spark plugs, maybe ive forgotten something...

Ive tried testing o2 sensor on oscilloscope and it seems fine except the frequency is about 0.5Hz, from what Ive learned it should be around 1-2Hz.

But what bugs me lately is reading from torque. One screenshot was made during idle, the other at steady throttle at about 3500rpm.

Can you tell me how is vacuum measured in my car? I have map sensor, is that responsible for measuring vacuum? If yes, this reading of 40inhg seems wrong to me, it was 16.9 like two months ago. Or did i miss something and im reading something wrong?

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Map reads normal atmospheric pressure at key on engine off.

Yeah ive forgot that ive checked fuel pressure during driving and at shut off, everything seems fine, about 55 psi constant pressure, no drops during driving or at shutoff, only like half an hour for 10 psi drop after shut off.

Ive also checked injectors on injector tester, they seem fine, no codes or problems other than that, other than two ocassions ive gotten po341 and po132 at the same time heres the freeze frame
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Car sometimes cranks longer than usually when starting.
Ive tried relative compression test with oscilloscope and the current peaks seem fine, all the same.

Unless your app cannot interpret the data correctly i would say test the map sensor with a known amount of vacuum from a hand vac pump like a mighty vac hand pump. And check what it reads as you slowly apply vac. If numbers dont match ~+- 2 psi then i say it needs replaced

Thanks for the reply, I waited longer to respond because at the time i have already ordered new map sensor. Yesterday i received it and installed it and the problem still remains the same. Nothing has changed, what else could be wrong, do you have an idea?

Hello there,

I had one of those at my shop last year and it's problem was a stuck open canister valve.

You can just strangle the hose which goes to intake manifold to check whether or not
STFT changes.

I hope it helps.

I dont think that is it, ive already checked purge valve and it works, as i could test it, also, there would be check engine light on if it was stuck open.

But here is better picture of what is happening with the car and throttle body at low throttle positions


See what it is doing? It is moving throttle plate on itself, computer does that i think. Ltft is mostly on -12.5 to -14.1 while driving steadily.

youtu.be/-qSveWyEwJU

Hello again, Ive changed O2 sensor for a new one, MAP sensor for a new one, fuel pressure regulator for a new one, this one is rated for a lower pressure than the original one, I have yet to verify for this weekend what pressure it really works on, it is rated on the web page I ordered it from for 3 bar pressure, as the original one is 4 bar, but still, the car seems to stabilize to the old habits, fuel trims in negative above ten.... It drives better now, but seems to me fuel trims will get back to "normal". Which is high negatives... At first, after changing fuel pressure regulator, values started to go up, sums of stft and ltft were at about zero percent, but looks like after few days of driving, they tend to stabilize to high negatives again.

Can you graph maf / map/ throttle iac / injector on time in ms. Also do you have a scope and do fuel trims change from idle to 2500 rpm? Do they improve or get worse

Hello there,

Factory fuel regulator is rated for 4 bar. 1.4L, 1.6L or 1.8L engine (what i have verified).

Ok, I have double checked your posts.

*This is a speed density engine (no Maf). Pulse width is base on Map sensor mainly.
*Map reading at idle: that should be something about 27 Kpa (instead of 39 at 96 degrees of ECT and 769 rpm).
*Can you confirm when engine off, what is vacuum reading?
*That number of 48 in/hg of vacuum at idle is strange (about 48 x 3.34 = 150 kpa baro pressure????). Can you confirm Baro pressure?

Freeze frame:
*ECT 33 degrees; about 6% throttle; about 978 rpm (ok, engine is warming up, but 51 kpa and 48 calc load??). Unless you have A/C On and any gear, this is not normal.

I asked you about evap valve in an earlier post. Are you sure it's ok?
Have you ever heard any strange noise (creaking) in tank area?

I think you still have a vacuum leak and that could be an internal one (brake booster hose (not related to negative fuel trims), evap leak (related).

Edit: By the way you must test evap system while engine is running

Thanks for your reply! No i dont remember ever hearing creaking from the fuel tank...


Here are my latest pics from obdlink, disregard some 0.0 values, it cannot read some sensors in the car or there are no sensors to read from...

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What seems suspicious to me is the intake air temperature as outside is around 1 degree centigrade and it reads 30 degrees....

Do you see anything else what looks suspicious to you? This was recorded on idle, cold engine, or in key on engine off.

Yes the original fpr worked at 4bar, this replacement one works at 3.6-3.7 bar, Ive checked it today, during driving too. Keeps steady at that value.

What are the consequences of this pressure drop, are they good or bad or none?

I cannot see any suspicious PID, except the one you mentioned. I would check not only sensor, but circuit integrity.

About the question of different fuel pressure, i would say you are changing factory conditions, which is not bad, but you are adding new variables in the equation (which decreases posibilities to get straigth to the root cause).

In general speaking, in closed loop the PCM is able to adjust the amount of fuel based on O2 sensor (i'd say no issues there). In open loop PCM fires the injectors according to preprogramed tables.

For instance at WOT if air/fuel ratio is too lean, gases can become too hot, engine starts knocking and NOx emission can increase (temparatures above 2000 F)

Ive gotten a new intake air temperature sensor and installed it. It started out good, 6 degrees centigrade which was correct temperature for outside, as i warmed up the engine, the intake air temperature raised to about 24 degrees when engine was fully warmed up. I have turned it off and waited for a few minutes with engine off and key on, the temperature raised to about 27 degrees centigrade. Now I have shut everything down and left the car to cool down for about two hours. Then tried again. Starting temperature now was 28 degrees not 6 like I started last time... And it raised it to 30 degrees again in a number of minutes, that is where I shut everything down again and left it. It seems to me that the sensor was working fine and was operating normally, but within minutes, it broke again, showing wrong temperature... What could be causing this?

IAT rising after you shut the engine off is normal. You've got a ~90* motor under the hood and no airflow. 2 hours might not have been long enough for the engine bay to cool all the way down to ambient. Did the car run OK when you restarted it? If it did I'd just recheck the IAT PID in the morning.

Yep youre correct. It is working fine. IAT is not the cause for surging but it still happens at constant speed