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928 GT, accelerates fine, misses when cruising

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928 GT, accelerates fine, misses when cruising

 
Old 11-03-2018, 03:42 PM
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macerl
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Default 928 GT, accelerates fine, misses when cruising

1990 928 GT: accelerates smoothly; however, misses a lot when cruising at highway speeds.
Recently put in new ignition wires, caps and rotors.
I presume all electric connections are OK if acceleration is strong and steady.
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Old 11-03-2018, 05:09 PM
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Marti
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Manifold air leak , resulting in lead conditions at cruise.

Under acceleration you get an extra squirt of fuel removing the lean condition
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Old 11-03-2018, 05:12 PM
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Originally Posted by macerl View Post
1990 928 GT: accelerates smoothly; however, misses a lot when cruising at highway speeds.
Recently put in new ignition wires, caps and rotors.
I presume all electric connections are OK if acceleration is strong and steady.
Was it running fine prior to the change?
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Old 11-03-2018, 05:23 PM
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When were the injectors last cleaned, balanced and re-o-ringed?

When was the mass-air sensor last rebuilt.

When was the O2 sensor last replaced?
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Old 11-03-2018, 05:42 PM
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1. was experiencing the missing before change. That is why I changed all the ignition wires/caps/rotors on advice of my mechanic.
2a. O2 sensor replaced just a few months ago along with knock sensors.
2b. MAF rebuilt about 2 years ago.
2c. unknown on injectors. will have to hunt through PO's receipts.

I am confused on why it would work fine under stress of acceleration, and miss when cruising along without stress.

Thanks for the responses. I will look through previous maintenance paperwork.
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Old 11-03-2018, 09:40 PM
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Has the LH computer been rebuilt?
Has the Crank position sensor been replaced?
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Old 11-04-2018, 11:51 AM
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Originally Posted by macerl View Post
I am confused on why it would work fine under stress of acceleration, and miss when cruising along without stress.
If your acceleration is at WOT, the computer uses different set of static fueling maps, and the O2 sensor is not taken into account.

While cruising, the computer uses another map, and where the O2 sensor is consulted to making additional fueling changes to keep the mixture around 14.7:1 air/fuel ratio.

According to your clues then, something could be wrong with the cruise system, including a faulty O2 sensor.

It can also be that the WOT acceleration and richer maps are masking something globally wrong such as an intake leak.
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Old 11-05-2018, 02:36 AM
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I note you didn't refresh the spark plugs. Start there. Always the easy simple stuff first.
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Old 11-05-2018, 03:55 AM
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Originally Posted by macerl View Post
I am confused on why it would work fine under stress of acceleration, and miss when cruising along without stress.
To expound upon what SwayBar wrote:

At a steady level cruise at ~60 mph you are using less than 10% of the maximum output of your engine. The amount of fuel and air entering the engine is minuscule. Any cylinder with some small source of additional (unmetered) air or additional fuel will have an air/fuel ratio inconsistent with good combustion.

On the other hand, when you are accelerating a small source of 'extra' air or fuel will be proportionally much, much smaller since a lot more metered air and fuel is required.

This is why I asked you about fuel injectors. Old and unserviced injectors can have poor spray patterns or can leak when they should be passing no fuel. At low output requirements (i.e. cruise) either condition can cause one (or more) cylinders to function poorly.


Last edited by worf928; 11-05-2018 at 01:14 PM.
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Old 11-05-2018, 11:08 AM
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Thanks. I will discuss injectors with my mechanic.
And I did put in new spark plugs with the ignition wiring and rotors/caps.
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Old 11-05-2018, 12:04 PM
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LH sent to Specialized ECU Repair in Aug 2015.
MAF sent to Injection Labs in June 2016.
2 x knock sensors, crankshaft sensor, ISV, Injector O rings, throttle switch by my mechanic this past summer 2018.
- (this was all done working on a idle issue, which seems to have worked).
Spark plugs, ignition wires, caps, rotors this past fall 2018.
- (to address engine missing at cruise, which obviously has not worked.)

Unrelated: re-upholstered front and back seats; new mufflers; new master and slave clutch cylinders.

1990 928GT - black/black
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Old 11-05-2018, 12:47 PM
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Would access to a hammer or shark tuner assist in diagnosing this issue?
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Old 11-05-2018, 01:27 PM
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First, is there anything non-stock about the engine on this GT?

Second, I really hope you aren’t paying your mechanic to learn how to work on 928s.
Originally Posted by macerl View Post
Thanks. I will discuss injectors with my mechanic.
Do so. Since the intake was off he must have done *something* with the injectors. Find out what.
Originally Posted by macerl View Post
LH sent to Specialized ECU Repair in Aug 2015.
Never heard of “Specialized ECU Repair.”
Who are they?
Where are the located?

Originally Posted by macerl View Post
Would access to a hammer or shark tuner assist in diagnosing this issue?
It might. It might not.
The ECU tests in my inspection guide (see my signature block) and a multimeter will do some of what The Hammer does. You could do the guide’s ECU tests yourself. They will reveal interesting and actionable data if you do them yourself.
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Old 11-05-2018, 01:51 PM
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Please describe what you mean as misses?
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Old 11-05-2018, 03:19 PM
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Mechanic is Stan Kostewicz in Orlando, FL. Recommended to me as 928 experienced by local 928 Owners Club. He has several current 928 customers.
Specialized ECU Repair, Oakland Park, FL; https://www.ecudoctors.com/; referred by my previous mechanic in Tampa area, Leo Leal, German Specialties.
MAF repair by Injection Labs, Inc; Boulder, CO; http://www.injectionlabs.com/massmeter.html; referred by my previous mechanic in Tampa area, Leo Leal, German Specialties.

When cruising I can feel a series of hesitations in the rhythm of the engine. I described them as misses to Stan, and on a test drive with him he confirmed. They are not completely regular, and can range from one every few seconds, to several per second.
They are not sufficient to dramatically effect cruising speed. They occur with or without cruise control and with or without AC on.
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