Need suggestions to chase rough running condition

ArtBasement

ArtBasement

Erwin Timmers
Since last year my X has been running rough on start up, both cold start as well as hot restart. It sounds like it's on 3 cylinders, but there is no single cylinder out. After 5 minutes of this, it seamlessly smooths out and runs fine.

If I disconnect the spark plug leads to cylinders 1, 2 or 3, I hear no change in running, but if I disconnect #4, the engine dies pretty much right away. I have spark in all plugs, and plugs are clean. I hooked up a fuel pressure gauge, and it seems my fuel pressure doesn't go above 40, is that low? Injectors were cleaned professionally about 17 years ago, or 15K miles. I can't think of any other condition that would resolve itself in about 5 minutes.

Ironically my Nissan has a sticky lifter which also takes about 5 minutes to quieten down...
 
AFAIK, the fuel pressure should be between 36 and 40, with the engine running and vary somewhere between 36 to 45 when changing from idle to higher revs. On my car, if the fuel pressure at idle is above 40, I get some rough running and some smoke from too much fuel.

How old are the spark plug leads? May want to check for resistance in the leads, as it sounds to me that you might have a weak lead or set of leads. Resistance can vary based on quality and material of spark plug leads.
 
I'd download a copy of the FI troubleshooting guide, but off the top of my head I would say maybe check the coolant temp sensor per the guide for the proper resistance when cold.

Regarding fuel pressure, it should be 36 psi +/-3 psi with the vac line disconnected from the fuel pressure regulator and 28 psi during normal running conditions.
 
Could be the aux air slide circuit, or the cold start circuit, since those are both 'timed' to engine temp/warmup modes, or wiring/component connectivity issues with either. The EFI coolant temp sensor would be another item, as suggested. Pretty much need to work through the EFI test guide to be sure, as Dan also suggested.

The X1/9 regulator has a base value of 2.5bar, or about 36psi as Dan stated. That would be with the vacuum disconnected. With vacuum applied (idle) it would be closer to 2bar/28psi. Pressure increases with load, so as vacuum decreases, pressure increases.

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Thanks for all the suggestions. It helps to go out with new suggestions. Then found that I still had a set of wires. Changed them over and condition fixed; Thanks!!

Still can't figure how that works in theory. Does some warm-up time improve the connections? Resistance in the wire would go up when warmer, I would think. BTW I measured about 8K ohm on my old wires and the new ones are at 3K. That couldn't be the difference could it?

Off for a drive...
 
ArtBasement said:
Since last year my X has been running rough on start up, both cold start as well as hot restart. It sounds like it's on 3 cylinders, but there is no single cylinder out. After 5 minutes of this, it seamlessly smooths out and runs fine.

If I disconnect the spark plug leads to cylinders 1, 2 or 3, I hear no change in running, but if I disconnect #4, the engine dies pretty much right away. I have spark in all plugs, and plugs are clean. I hooked up a fuel pressure gauge, and it seems my fuel pressure doesn't go above 40, is that low? Injectors were cleaned professionally about 17 years ago, or 15K miles. I can't think of any other condition that would resolve itself in about 5 minutes.

Ironically my Nissan has a sticky lifter which also takes about 5 minutes to quieten down...
Click to expand...

So you have one good cylinder and its number 4 have you done a compression test when cold to see what the other three are doing?
 
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