My 2000 XK8 with 58,000 miles has started to periodically stumble like it's starved for fuel immediately after starting.

It will stumble badly but not stall, for 20 or 30 seconds. If I try and ease some fuel to it while it's stumbling there's no effect. After about 20 seconds of rough idling it will settle into a proper idle and behave fine for all hot restarts. This happens about 3 or 4 times a week.

On my Northstar powered Caddy that symptom (along with a periodic backfire) was a sure sign of a failed Fuel Pressure Regulator. Rock auto dosen't even stock a FPR...

Any thoughts here?

 

Your car does have a fuel pressure regulator on the right rear of the engine at the head but on these cars the throttle body is a more likely issue.

I would suggest that you clean the throttle body, the MAFS and the part load breather before changing parts. A $5 can of TB cleaner could be a simple solution.

 

Good suggestion, I'll work it over with cleaner and some Seafoam as well then report back.

 

I have a similar issue but not during a cold start. Starting the car after it has been sitting for about 45 minutes, it will stumble and miss for 30 seconds or so before smoothing out. Occasionally this will cause the check engine light to come on. My mechanic suggested a can of Sea Foam and that seemed to help a little but it still happens. I also noticed a clicking sound from the front right side of the engine compartment that happens when the kley goes to the on position before starting. Not sure if these things are related.

 

Well...I cleaned the MAFS (It looked clean already) and made sure the sloppy fitting shunt from the air cleaner to the throttle body was sealed better. I'm pretty good about Techron/Seafoam in general.

Where is the part load breather?

Still have the problem periodically. Feels like it's starved for fuel until system can catch up. Happens on cold or got starts Happens 1 of 10 times and if it does it once it will do it several times, then stop.

I will check the maintained fuel pressure in the F/I system. (I think it might be a failing fuel pressure regulator but they are $150 - too much for a blind test)

Is there an fitting on the system to attach a pressure tester?

 

How many miles on your engine? Have the spark plugs ever been changed if you're around 100K miles? The condition you describe with cold start hesitation and/or rough running then clearing when warmed up is more likely with spark plugs, or next, O2 sensors not heating up normally.

The MAFS, fuel pressure regulator and other items mentioned will all set fault codes if they're operating out of range. So, if you're not seeing a fault code, you're just shotgunning parts replacements and, as mentioned, can be an expensive proposition.

Both poor condition spark plugs and O2 sensors beginning to fail are not going to set a fault code on cold startup, but still cause the problem, and both plugs and O2 sensors, as they start to fail can improve performance within acceptable operating conditions fater warmup.

 

Hello Steve.

I should have this in my signature - 55K miles MY2000.

New plugs last season. No fault codes. Happens intermittantly on cold as well as hot starts. I might go a week between events.

It feels like the sort of probem that will get progressively more frequent.

Fuel pressure access point?

 

The breather stub is located just above the oil dipstick on the valve/cam cover http://www.gusglikas.com/AutoRepairPlugReplacement.htm The only location that I know of for testing the fuel pressure is on the fuel rail. I see that SeaFoam is the cleaner of choice on this forum I seem to favor BG 44K fuel additive. I must caution you that excessive use of fuel additives is not good for the car and is not recommended. I agree that the plugs should be looked at and are often the cause of poor idle and startup. As for the 02 sensors I had little experience with them but with a good computer based OBD reader you may see variations that may help. I have seen a few garages use them. Check the plugs and clean the TB.

 

Yes, agree strongly with Gus on the fuel additives. Any of the popular ones, Chevron Techron is my choice, can be over used, too much concentration. This will cause rough idling, poor cold start performance, and wreck havoc on in tank fuel pumps, rubber seals etc. Just make sure the concentrations are followed.

 

Thanks, Gus. I'll check the breather stub for a cracked or loose fitting or something similar.

The fuel rail is typically the place to check/relieve pressure, I'll look for a fitting.

You are also right about over use of cleaners.

 

I used the Techron and it seemed to do the job I am just not that familiar with the SeaFoam. I guess I am in a rut I have been using BG 44K for years and has not let me down. The Techron was added to my Rav4 and it did help out. Might try it again.

 

Gus,

I was introduced to Techron with the purchase of my 1988 BMW M6. Techron was called out in the handbook and a requirement of the warranty. Don't know how they would have verified that I used it, except in those years, fuel supply was pretty unregulated compared to today, including heavy concentrations of sulfur, etc. Maybe carbon buildup would have been significant without its use.

 

Well my last adventure into the cylinder washing I found that most all were caused by a poor presentation of fuel to the cylinder and I believe it. I will continue to be vigilant and do what I can to be sure the engine is running clean. Whatever works best for me I will stay with.

 

I've already come to admire that about you Gus. You're a man of conviction and integrity with a good technical compass.

 

OK! Are you trying to be Nice???? Only kidding! Thanks!

 

Not nice, realistic.

 

I will swear by the BG 44K!! It works!

 

Fixed.

It was the Fuel Pressure Regulator. It was causing an over rich condition across both sides of the fuel rail and tripping codes P0172 AND P0175. When they appear together the JTIS recommends the FPR among a coule of other things.

I bought mine at autopartswarehouse.com for $149. P/N JLM 21358 also P/N W0133-1656916 (I used the second one for an internet search)