No start when at warm temperature Topic is solved

[afranke]

The “injector relay above the radiator” is a 7/9-series thing. On an 850 the relevant piece is relay #103 in the main fuse/relay box. It can fail on the ECU/injector side while the pump still runs, so “I hear the pump” doesn’t clear it.

Do this when it won’t restart (takes 10 minutes):

1. Spark vs fuel:
   • Clip a spare plug to a plug wire, ground it, crank.
   • No spark: most common culprits are crankshaft position sensor (CKP), ignition power stage (amplifier on the fan shroud), coil, or cam sensor.
   • Good spark: try clear-flood (pedal to the floor while cranking). If it then fires, you’re rich/flooding.
2. Tach needle check: On 850s the tach usually twitches while cranking if CKP is alive. No twitch = suspect CKP even if it ohms “fine” cold. CKP heat-soak failures are very common.
3. MAF quick check: Unplug the MAF and try to start. If it lights off, the MAF/IAT input is skewing fueling hot.
4. Fuel pressure & flooding checks:
   
   • Pull the vacuum hose off the fuel pressure regulator (end of the rail). If it smells like fuel or is wet, the diaphragm’s leaking, a classic hot-soak flood.
   • If you have a gauge: you want ~43 psi with the vac line off, a bit lower with it on, and no rapid bleed-down after key-off. Rapid drop = leaky injector/check valve → hot restart issues.
5. EVAP purge valve stuck open: Clamp or unplug the hose from the purge valve to the intake and try a hot start. A stuck-open purge dumps vapor and mimics flooding.

Parts/tests in the order I’d attack them:

1. Swap/repair relay #103 (or open and reflow the solder joints). It can power the pump yet drop ECU/injector feed when warm. Cheap and easy, might as well eliminate it.
2. Crankshaft position sensor (top of bellhousing). If tach doesn’t move, no spark, and injectors are dead, just replace it, heat-soak failures are textbook on these.
3. Ignition power stage (ignition amplifier) on the radiator shroud. Another common heat victim; quick to swap and often original.
4. Fuel pressure regulator if that vac hose smells like fuel or pressure bleeds down fast.
5. MAF if unplug-to-start test is positive.

[Pavel9]

This is turning out to be a bit of a mystery...

Taking Vova585's advice, I checked the coil - it's the single coil and distributor design, not coil-on-plug. I could not get ohms readings on the primary and secondary sides of the coil for some reason (cleaned everything down to bare metal) but was able to show spark at the coil secondary port using an old time spark tester. There was spark when the engine was cold (that would lead to a running state) and also when the engine wouldn't start when warm. So, the coil should be good.

With warm engine, I checked the fuel rail Schrader valve and found ZERO psi using a simple tire gauge! Could this be a clogged fuel filter - the original was still in place from 1997? Replaced the fuel filter. No improvement in starting when warm. With a cool engine and normal start up, the reading was 38 psi at idle.

What's left? Could the fuel relay (#103) be failing when the engine is warm, but still functioning with cold start up? Next I'll find out how to bypass the fuel pump relay.

[Pavel9]

afranke, just read your comments, will digest overnight...

[Vova585]

"0 psi" might be the issue..
AFRANKE wrote a great post. Respect for his knowledge. I doubt that issue is leaky injector or leaky check valve. If that would be the case- you would have problems on hot/cold regardless(until you finally prime the system out of all the air and pressure is at minimum required for this car to start).
850s gurus, does this car has dedicated fuel pump relay? I can see the case where relay is weak and when getting hot the winding is internally shorting, or control side is being lost(wpuld need a wiring diagram to find out how it is controlled through the ecu or some relays). In case you found that it is 0 psi and if this car has relay(#103 as per your reply above) I would use a fused jump wire between terminals 15 and 87 of the relay(remove relay when car is not working, find a good piece of wire and put 10-20 amp fuse in the center of the wire(auto zone will sell those premaid if dont want to do yourself), get 2 flat pins of proper size to go into 15 and 87 terminals in place of relay and crimp them. Now you created a bridge and it will supply power constantly to the fuel pump. If your car is running like a champ with this device-issue is either relay or control side of the relay.

[Vova585]

It should be similar to 850 would be my guess. Impressive work by CN90 member.
DIY: 1998 Volvo V70 Fuel System Troubleshooting Tips https://share.google/zlcAdecYOUQHdEk6G

[Pavel9]

A little progress... removed the #103 relay with engine in warm-will-not-start condition and jumped the 15 to 87 terminals in the panel. Pressure at fuel rail Schrader valve now so clearly the fuel pump relay is faulty. With the jump, the engine "tried" to start but failed. It stumbled a few times and died. An improvement of sorts...
The battery was low and the starter was unenthusiastic, so I'm charging it up now. The cheap battery was showing about 11.5 volts on my cheap code reader.

Several details, probably only peripherally involved, should be mentioned here:
1. After the fuel filter replacement, the cold idle is much improved - no more almost stalling (down to 700rpm) before recovery to about 1000 rpm. Engine is smoother at cold idle.
2. I noticed the white plastic base where the relays plug in to the fuse panel is cracked in several places. I will glue in some kind of support eventually.
3. The only code indicated is P0410 - seems unrelated to this warm restarting problem.

[Pavel9]

The sleuthing continues...

Clamped off the tubes to the "evap thing" under the battery... no change.
Checked the tach is working... it is.
Pulled the wires to the MAF sensor... everything returns to "normal". So the sensor is dirty or bad or the connections are faulty. Once the engine was running, if I plug the sensor back in, the engine dies.

Cleaned the mass airflow sensor and now the vehicle won't start warm or cold! So, it seems I need a new sensor.

Also, I noticed the "Check Engine" light doesn't come on when the ignition key is turned to the first stop... bad bulb?

My code reader now shows P0103 (Air flow circuit high input). This code did not show originally. Is it now showing the code because I unplugged the sensor or is the sensor bad.

[abscate]

That code is caused by pulling the sensor wire while running.

You’ve discovered no fuel pressure is your warm no-start cause, so it’s ignition switch , fuel pump relay, wiring, fuel pump on the diagnostic tree.

You fault trace backwards, do get a good battery in the car, then create the warm no-start, then measure voltage at the fuel pump back at the connector in the trunk. Work your way forward to where the voltage is found again

If there is voltage at the pump, and no fuel pressure, you’ve found it!

[Pavel9]

Thanks abscate... but why does the vehicle start, cold or warm, with the MAF disconnected, but not start with the MAF connected?

Surely, if the engine starts with the MAF disconnected, the fuel supply system is working. I'm confused.

[Vova585]

Is it possible that MAF is on 5v reference and due to internal fault it "brings the system down"? Would be great to find specs for resistance in Maf and compare to what you have. Or find a used oem and swap in and hopefully all is great!.