Quote:
Originally Posted by Old Mechanic
Yep, the first EFI Z cars (1975-76 federal) did not have EGR valves. The intake manifolds were super clean. When they added EGR everything inside the manifold downstream of the egr entrance was covered with a tar like residue, except the intake valve stems where the injectors sprayed fuel.
regards
Mech
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As a z-car specialist going back to my time at "z-shop of Miami" in the 80s, I agree with your assessment on non-egr engines being clean of the egr fouling but those non-egr engines still fell victim to fuel deposits on the back of the intake valves which would grow until they eventually looked remarkably like what you would find on an egr equipped engine.
The mechanism is explained below.
In my experience, all of the injected nissan "L" engines (L28e and L24e) suffered intake valve coking and I made a good portion of my living removing deposits that looked as if someone had impaled a Hershey's kiss on the valve stem and slid it down to the back of the valve.
That's a lot of accumulation and, once it get's a foothold, even the pcv oil mist will start sticking regardless of the detergent fuel being sprayed directly onto it.
On this engine family, my theory is that the initial fuel deposits are very "rugged" and stick with much enthusiasm to the point of not being removed by the thermal shock of cold fuel on a hot valve and also being immune from the fuel detergent.
Once adhered, the fuel deposits insulate the area where the fuel is sprayed and the accumulation accelerates.
Once this happens, the pcv oil mist adheres to the fuel deposits since the high temperature that would normally embrittle it and cause it to chip off in chunks is no longer there.
You end up with a large, black mass on the stem that threatens to fully choke off flow.
Once begun, the above mentioned insulating allows both fuel and PVC mist to accelerate it's rate of accumulation.
Yes, it is much more gnarley with the addition of egr skank to the mix but non-egr versions of this engine family are not immune if the proper conditions are met.
Driving these engines insanely hard seems to be the best "medicine" and the reason we saw rarely saw this on turbo engines is because the type of driver who is motivated to buy the turbo version of the ZX would be more likely to drive them "insanely hard".
This statement is only directed at this engine family though similar things can happen if the same conditions for causation are met.
Also know that fuel composition is constantly evolving which will vary the nature of deposits or lack thereof throughout the timeline.
The newer gdi engines have their own set of issues and causation though much is caused by pcv oil misting and valve timing schemes to encourage egr without needing a dedicated egr system.
I feel a better designed pcv system in gdi engines that can prevent any oil in mist or liquid form from reaching the intake tract while still passing the other products of blow-by through the engine will work best at reducing or eliminating the deposits we are currently seeing.
Contrary to popular belief, egr can enhance efficiency at low load cruising because the lower combustion temperatures allow for more spark advance before the onset of pre-ignition.