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Hard start after fuel injector upgrade

Tbone289

NAXJA Forum User
Location
Missouri
A couple of weeks ago I did a thorough cleaning of the throttle body, IAC and all grounds on the XJ, reset the PCM and afterwards it was running smoothly and starting nearly instantly.

This morning I R&R'd the injectors (upgrade to 784s), reset the PCM again, and now cold starts are are sluggish, taking 3-4 seconds of cranking. If I shut it off and restart it soon after (within a minute), it starts as it was before. It runs really smooth with the new injectors, but cold starting has definitely been negatively affected.

Should I wait for the PCM to learn for a while, or is there something I should start looking into? I'm guessing that it might be draining back fuel as it sits, but I haven't rented a fuel pressure gauge to see how quickly it bleeds off. I've owned this XJ for ten years, and it's always cranked over really quickly, even after sitting for a long while (weeks or even months at a time).

The only thing besides the injectors that was disconnected and reconnected during the procedure was the battery and the IAC. The battery, which is only a couple of months old, has a full charge.

Any help would be greatly appreciated. TIA
 
Yes the PCM needs time to adjust, what have you done to the motor as those are pretty large injectors!
 
Nothing. 784's are the EV6 replacement 4-hole injectors for factory XJ injectors, '99-'01. I have a 2000.

I don't think this is a PCM re-learning issue. I'm going to rent a fuel pressure gauge to see if it's bleeding off after shutdown.
 
rented a fuel pressure test kit this morning. Here are the results:

Pressure after priming 8-10 times: 48 PSI
Pressure at idle: 46.2 PSI
Pressure leakdown: 30PSI after 28 minutes

Obviously I have pressure bleeding off fairly quickly and my idle pressure is within spec (49.2 PSI +/- 5 PSI), so at this point I needed to determine if the injectors were leaking or the check valve in the pump was the culprit. I took off the throttle body elbow and could smell a faint whiff of fuel, but probably not enough to indicate the injectors were leaking. For good measure, I disconnected the injectors and pulled the rail, flipped it over 180 degrees and primed the rail back up to full pressure again. There wasn't a single drop of fuel from any of the injectors.

I would like to be able to clamp off the fuel line to further test leakdown through the pump check valve, but I don't have a proper adapter for that. The FSM indicates that leakdown to 0 PSI after cooldown is normal, and that the pump should prime the fuel back to normal pressures in 1-2 seconds before starting. I have to prime mine at least 4-5 times to get it back to normal pressure.

I would like to conclude at this point that I need a fuel pump module. Is there anything else I should test?
 
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The FSM indicates that leakdown to 0 PSI after cooldown is normal, and that the pump should prime the fuel back to normal pressures in 1-2 seconds before starting. I have to prime mine at least 4-5 times to get it back to normal pressure.

I've always heard that the manifold should retain it's pressure. I always have to bleed mine down before any work! Do you have a scanner to check your fuel trims?
 
Yes, I do, and I haven't checked them yet.

The fuel should always remain in the manifold, but it is not abnormal for the pressure to drop.

From the 2000 FSM. See boldfaced text:

36837344532_8a6ecf3c9d_o.jpg



I'm not convinced it's the fuel pump either... So, I put new orings on the original injectors and put them back in to see if operation returns to normal. 5 minute job, why not?
 
Thanks guys for your comments here! I sent the injectors back for a refund, and not sure what my next steps will be. My stock injectors are fine, so this isn't really high on the priority list. I'm pleased at this point that I'm not dropping my tank to install a new fuel pump due to a bad check valve.
 
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