Erratic starting mostly when hot but cold sitting 855T 96.

[rguzz]

If the car sits more than a few days (cold) starting is rough smooths out after a minute. This is consistent. Lately hot starts are problematic, runs fine, stop for gas then cranking and sputtering when attempting a start. Again runs fine after a short bit. Hop out, run in the house, back out and same result. This has happened over the years rarely but currently now commonly. I've done tons of stuff, half in an attempt to chase this down but mostly as maintenance. Would someone remind me by listing, in order of likelyhood, possibilities? I have ideas of course but if I had a list that would help me stay organized as I look further, and thanks. I should add I have evap codes sporadically (not PO455) but have been ignoring them for so many years I don't even remember what they are. !! None now.

[Chuck W]

ECT sensor and/or connector.

[WhatAmIDoing]

Seemingly temperature dependent? ECT sensor, then fuel pump relay, then maybe purge valve stuck open.

[rguzz]

Yep, thanks. I suspect the wiring to the CTS is likely to be the main culprit. CTS a year old and oem Volvo. Easy enough to swap out the FPR I have numerous spares of both relays. The lack of CEL's narrows the field a lot, thanks for the reminders.

[yanga001]

Check the head gasket. One way to see if its combustion is pull the thermostat, start the car and tilt the bottle such that the overflow tube (small tube) is fully immersed in liquid. On startup you may see some bubbles but it will stop. If you see bubbles consistently you may have a slight leak into the combustion chamber. I had this exact issue on my late v70 since purchase, and it accelerated after a coolant pipe popped off. Turned out i had a very very slight leak since purchase and it got worse as time went on. It got to the point where turning it off after having it on resulted in 10-15 seconds of rough running until it smoothed out.

In terms of check engine lights, i only got misfire codes when it got real bad. Otherwise no codes, sometimes bad o2 sensor codes but that may have been caused by a leaking exhaust manifold.

I was in the same boat where it was tons of work to get the engine happy, fully synthetic only, etc.

When you say it sputters does it crank, fire and then run hard?

My theory on why it affected it more on temperature swings was expansion and contraction of the aluminum head in regards to temperature.

Another easy way to eliminate this is run the engine till its up to temp, leave it for 15-20 minutes and check pressure. If there is little to know pressure then you may have a leak into the combustion chamber or elsewhere.

Looking forward to hearing this theory proved wrong but i hope it helps you narrow it down.

[rguzz]

Yes, the HG can be tricky to pinpoint early on, thanks for the reminder. For what it's worth, burnt valves x 2 and HG replaced by reputable shop maybe 5 years ago, coolant levels always normal, and runs great after a few seconds of rough running. I like your method of leak testing! Compression also is perfect, though not foolproof for small leak, I know. I'm working my way down the list as we speak.

[scot850]

CTS = ECT? The unit may have been replaced but the wiring and all the connector probably were not. I would check there for sure.

Fuel pressure? Have you checked that at all? 43 psi is normal and maybe a bit less when running.

Fuel filter last replaced?

Neil.

[rguzz]

Safe to say, simple solution here, swapped out fuel pump relay and so far so good. Almost every other thing on the list has been addressed over the last couple of years, and no codes narrows it down quite a bit. Harness to ECT rebuilt and looks very clean. Time will tell of course, as we head in to the heat of summer. Yes, fuel pressure is good. On to bigger spring projects. Thanks.

[scot850]

Always like it when it turns out to be a simple issue! Can't say the symptoms are what I would typically expect for a fuel pump relay so really good catch.

Neil.