I thought I might start this thread in the hope some people may find it useful. It appears from my personal experience and time on this forum that there are at least 3 different behaviours on this engine that many people describe with the same terminology, yet they are very different. So I will describe them each below, along with what I believe can be done to rectify them. This is just a start and needs more work - but I hope one day it could develop into a howto...
Problem 1: No power below 2000rpm
The exact RPM will vary, but in most cases this fault is identified by an extreme lack of power, then as the revs gradually rise the engine bursts into life at a specific RPM and produces full power, typically somewhere between 2000 and 2500 rpm.
Cause 1: EGR valve is sticking. This is sometimes accompanied by a spanner-through-car or engine light on the instrument panel, which extinguishes once the engine starts producing power again. Fault codes relating to EGR flow/performance may be present. Unfortunately in the early stages of this fault, there may be no lights or fault codes present.
Solution 1a: Remove and clean the EGR valve as described in the howto. If this doesn't work then further diagnostics and potentially a new EGR valve may be required.
Solution 1b: The EGR valve can be disabled in software and optionally 'blocked up' with a metal blanking plate. The software part can be done by a tuner, and may also include a performance remap. Note: Blanking without disabling in software will cure the fault but result in a permanent Engine light on the instrument panel.
Cause 2: Swirl valve mechanism stuck/broken
I think most people on the forum are familiar with this one. The bar linking the swirl flaps on the intake manifold will often be detached. Typically this one will not cause any fault codes or warning lights.
Solution 2a: Replace the complete intake manifold (time consuming and expensive), or effect a temporary repair as described in the howto's.
Problem 2: Momentary hesitation at around 2000rpm
When full throttle is applied at around 2000rpm the engine will offer a surge of power, followed by a momentary loss of power for 0.5 to 1 secs, then resume normal accelleration. It is as if you had lifted your foot off the throttle pedal for a moment and then re-applied it.
Cause 1: Boost pressure control solenoid slow to operate or split vacuum pipes
Solution 1a: Check condition of, or replace all vacuum pipes between the vacuum pump and boost pressure control solenoid, and the vane actuator in the turbocharger.
Solution 1b: Dismantle the boost control solenoid and clean the filter as described in this thread: http://www.vectra-c.com/forum/showth...dti+hesitation. If no cure, then try replacing the solenoid.
Cause 2: Variable vane actuator or mechanism sticking
Solution 2a: Log data using opcom or similar tool and look for boost spikes. If boost spikes are present after solutions 1a and 1b have been completed, it may be necessary to have the turbocharger inspected and possibly reconditioned/replaced.
Cause 3: Software fault
Some software versions had the boost pressure tolerance (PCR Governor) values set to tightly. Although these values were ok when the engine and components are new, normal wear and tear which results in slightly sluggish operation of the VNT control vacuum/mechanical components can cause the PCR governor to over-compensate resulting in this hesitation.
Solution 3a: Later software versions appear to have a larger PCR Governor window size, which cures this problem. I don't know when GM changed this as I have only compared two cars - one was last updated in 2005 and had the 'problem values', the other was updated in 2008 and has the 'corrected values'. If you have the opportunity, get the software updated by a vauxhall stealer. Or a competent tuner should be able to change these values along with other useful changes such as egr disablement and a performance remap.
Problem 3: Turbo Lag and high boost threshold
This is not a fault per-se, but rather an inherent characteristic exhibited by all Z19DTH engines, that many people complain about even when the car was brand new. Basically there is very little power below 1500rpm (boost threshold), and when throttle is applied suddenly between 1500 and 2000rpm it can take a couple of seconds for any useful power to develop (turbo lag).
Cause: The high boost threshold appears to be caused primarily by the sub-1500rpm torque limiter values set in the software, which are very conservative and actually much lower than the Z19DT engine as can be seen in a side-by-side comparison in the screenshot below:
I am not sure why GM have chosen to set the limit so low on the Z19DTH engine - perhaps something to do with emissions? If it was to protect the DMF, I would expect both engine types to have equally low values. The second part of this problem - turbo lag - is caused by the values in the smoke limiter map.
Solution: A tuner will be able to change the torque limiter values to improve the amount of power available below 1500rpm. They will also be able to alter the smoke limiter map to reduce the turbo lag (perhaps less so on DPF equipped cars though). On mine I've altered both and the difference it has made is incredible - although I fear for my DMF and may back-off a little. But setting the 1000-1500rpm values to the same as a Z19DT should be safe.