M50NV intermintant hesistation

[Nick_S]

My car has been running for a weeks now basically flawlessly. However, it will occasionally run a bit rough and seems like it's not getting quite enough fuel. I initially replaced the 4 prong Bosch plugs with single prong Denso and that helped for a few days. It can be very random and first seemed to be a cold start issue. When it's studdering and you floor it, it smooths out and pulls hard. My relatively limited knowledge has me believe it's a fuel delivery issue. Thinking it could be either be the fuel pump which I believe to be OE, the FPR or the questionable injectors I soaked in seafoam and put new O-rings on possibly not firing or not firing consistently. Before I start throwing money at this, what's the best definitive way to test? I assume and actual fuel pressure gauge? I haven't located any vacuum leaks around the intake manifold.

To rule out some things, the following are all brand new that I'm aware could cause a similar issue.
-CPS
-O2 sensor
-spark plugs

Car is running a 92 M50NV with a 403 and Dinan chip.

[Jib05]

I have a similar problem, turned out to be one of the ignition coils going bad. There's a thread on it somewhere on how to check them.

[justin_l_]

Have you pulled the plugs to see the condition of them? It could be ignition break up.

^ Beat me to it

[Nick_S]

Yes when I changed out the plugs they all look fine and consistent. For some reason I hadn't thought of the coils, probably because I was told they were good, lol. I'll check into that. I could even swap my known good coils from my wagon to check.

[Nick_S]

Tried my other coils I had laying around. They were MUCH worse. Lots of sputtering and back firing so I tossed them, lol. Ran out of time but I'm going to swap in the coils off my good running E34. We tested the resistance of the coils and they all within spec and pretty much dead even so I'm starting to feel they may not be the issue. Then again, my old CPS tested well within spec and it still was causing issues.

[TimKninja]

I had the same exact problems, my donor car had no coil pack cover and water got in to the spark plug holes. made the coil pack boots have a rust coloration on them.

I did the coil pack boots, plugs and the fuel filter and the problem went away.

The PO of the donor car has replaced the coils a month before the car got t-boned. Its really a good idea to do them.

[Nick_S]

I was reading a bad fuel pressure regulator could cause hesitation and surging. Thinking about replacing it, the injectors with rebuilt ones and new coil packs. About all that would remain is the fuel pump at that point.

[hilux30]

Yes it could, you can test it with a simple vacuum pump (I got the autozone one and still works) and a fuel pressure gauge to see about the pump...

[Nick_S]

Well today after swapping in my known good coils from my wagon and it acting the same. I decided to swap out DME's since I had a few extras around, before I started throwing money at it. Took out the 403 with the Dinan chip for a factory reflashed 402 and it seems to have cleared up the issue? Very noticeable power decrease and poorer throttle response without the chip but it runs super smooth and consistent now. I'll drive it for a bit before I confirm it was the issue but it seems to have cured it. Strange.