Poor running

[arf3rn]

1990 with 67k miles. Started having this little warble out the exhaust and occasional back fire when cold. When gets warm (closed loop I assume) runs like garbage. Dies. Won?t start till cools off. Paper clipped the code reader and got H-42. Anyone have this issue before? Thanks.

[secondchance]

https://www.zr1.net/forum/showthread.php?t=27718

At least for Bill, H42 was caused by low voltage at battery.
Based on my experience from way back when, did you check vacuum line from MAP sensor to the back of the plenum?

https://www.jerrysgaskets.com/hose-m...ew-90-95-13d4/

[arf3rn]

I did. Even put a new on in case I couldn?t see an issue. Still puzzled.

[secondchance]

Assuming you are new to ZR-1 (and you may not be), a lot of LT5s ended up with leaky injectors due to OEM injectors not fairing too well with ethanol found in gasoline.
Has the car run poorly since you got it? Was it running fine until recently? Some back ground info may be helpful.

[arf3rn]

Been running like a champ. Per history and receipts it got new injectors at 57k miles - was about 15 years ago and 10k miles. Car got less than 800 miles on it in 12 years. I got it last May and have put 2k on it so far. Would have more but for the last month been fighting this issue.
Put fuel pressure gauge on it last weekend and it looked good.

[RSLyrick]

Code 42 (ECM code) is supposed to be related to the ignition mechanism (DIS / ECM)

See the pages attached.

The DIS and the ECM interact with each other over a set of 4 signal lines (+GND) :

- The crankshaft signal (for engine position and RPM, provided by the DIS to the ECM)
- The Bypass line (ECM applies 5V to this line to tell the DIS that it is now in charge of timing management, after reaching 400 RPM)
- the EST line (which is a modulated signal provided by the ECM containing the timing information to be used by the DIS, a signal transition from high to low tells that the next coil has to fire. This transition happens sooner or later to adjust timing).
- The fault line reporting the following errors :

1) The ignition module detects an EST fault input on CKT 423 (EST).
2) The DIS fault input CKT 900 to the ECM is open (poor connection/connector pin).
3) The DIS fault line is shorted to system ground (pinched wire).

Code 42 appears if there is a short or an open on the Bypass line or / and the EST line. This automatically forces the DIS to run back on its own timing table (engine running then poorly) like it does below 400 RPM, instead of using ECM timing data.

So code 42 can be triggered by the following things :

- connection between ECM and DIS for the Bypass / EST line is open or shorted.
- The connectors are not properly plugged into the DIS or the ECM.
- The electrical interfaces managing the Bypass Line and the EST line are faulty (receiving interface for the DIS, transmitting interface of the ECM).

If your engine is starting fine, running fine for a while and get suddenly code 42, it could be the ECM / DIS warming up, having weak solder joints at the connector which open / close with temperature cycling.

There is a diagnostic connector for the DIS link. Insert an oscilloscope probe into port C (EST signal) and E (Bypass signal) referenced to a clean ground and look for a sudden change in the behaviour.

Below 400 RPM :

- EST (Port C) < 1.3V.
- Bypass (Port E) around 0V.

Above 400 RPM (running):

- EST (Port C) < alternating between 5V and 0V (everytime there is a transition from high to low, a coil will fire). It is, somehow, a frequency modulated signal.
- Bypass (Port E) around 5V.

The smart thing about this DIS module is that it outputs the crankshaft signal to the ECM and monitor the EST signal send by the ECM at the same time. If somehow, there is transition from high to low that don't make any sense in regard to the cranshaft signal (engine position), it knows that something is wrong.

A faulty crankshaft sensor could (with low chance) lead to Code 42 but I don't see the correlation with the engine warming up...

[tccrab]

My vote is on the Map sensor hose.
Check it first, then see if the Code 42 comes back.

'Crabs

[arf3rn]

My thought too. Changed it out with a new one from Jerry?s Gaskets. No change.

[secondchance]

May be disconnect map sensor hose and start the car. If no difference, may be MAP sensor gone bad.
You shorted the ALDL into diagnostic mode and after flashing ?12?, no more codes. Right?

[arf3rn]

I did clear it. I can?t remember what the process was. I pressed eng/met or trip/odo or one of those buttons on the dash.