Intermittent Drivability Issue - Mostly Warmer Weather

dnd2526 · Post by **dnd2526** » 10 May 2020, 14:51

Summary: Under light throttle (e.g. while cruising on a level road section), at idle and decelerating/coasting, the engine will buck as if fluctuating between power loss and normal operation. At this time, the exhaust smells funny (I don't know if it's rich or lean. How to tell?). This is intermittent, sometimes going hundreds of miles without occurring, or it can be persistent for days. Seems to be more common and stronger in warmer weather.

About a year ago, my daughter was driving my 850 as I rode shotgun. I kept getting the sense that it was occasionally bucking, almost like a weak fire rather than a misfire, ever so slightly. Over the course of a few hundred miles or so the issue worsened to the point where, while trying to maintain speed up a hill, the engine lost power so bad that I had to pull over and the engine stalled out. I waited a few minutes and restarted the car and was able to limp home. When I got out to get the mail, I could hear the fuel pump whining. I assumed I found my problem and installed a new fuel pump assembly and replaced the fuel filter. This seemed to fix the issue, at least for a week or two. Then, just like before, the intermittent stumbling and rough idle started, first infrequently but gradually becoming more common. While poking around under the hood searching for vacuum leaks, I noticed the intake tube had completely torn away at the throttle body, so I ordered a new one. At about the same time, I also discovered the air box thermostat had failed and the diverter flap was constantly diverting hot air into the intake. So, I simply fixed the flap in the closed (cold air) position. Once I installed the new intake tube and cleaned the spark arrester, the car ran well again...for a week or two. Then, as before, it gradually worsened. This time I removed the IAC valve and thoroughly cleaned it and made sure it was not sticking. Once again, this fixed the issue for a short time. I became so frustrated that I parked it for a few months. I considered the possibility that the MAF sensor was damaged by the hot intake air. I had noticed that even the K&N filter that the previous owner installed had the oil "cooked" away in areas (no longer red in color). So I verified the poor operating condition just prior to replacing the MAF sensor. After replacement, the car ran flawlessly again for about two weeks. Note: I did not disconnect the battery at all of these service points, so the ECM was not always reset. However, I have done this since the MAF sensor was replaced but the condition still exists. I also decided to discard the K&N filter and installed a common style Purolator air filter.

At this point, I am not sure what else to consider. I don't have the diagnostic tools to reliably monitor real-time data (I have a bluetooth dongle and phone app). I am wondering if the air intake temperature sensor may also have been damaged by the hot intake air. At this point, I'm just throwing money at it without knowing what the problem is. Any guidance is much welcomed.

Post by **misha** » 10 May 2020, 16:48

Since you have '95....you have OBDI diagnostic tool under the hood.
All you need is a pencil and a piece of paper to write down flash codes from A2(ecu) and A6(ignition).

Fuel pump relay (most likely)or main injection relay(grey one on fan shroud).
Since the problem occurs in hot weather...it's something which gets heat soaked(like relay).
Cap & rotor,spark plugs,wires should also be checked and replaced if needed.
Read the codes,write them down,clear them all and see which ones comes back.

Post written codes here.

Post by **rspi** » 11 May 2020, 07:46

dnd2526 · Post by **dnd2526** » 11 May 2020, 15:23

misha wrote: ↑10 May 2020, 16:48 Since you have '95....you have OBDI diagnostic tool under the hood.

Post written codes here.

Mine has an OBD-II port in the center console. There are no codes which is odd. It has never reported one for this issue. I will check the relays, though, since that is easy enough to do. Thanks for the pointer!

Post by **misha** » 11 May 2020, 15:38

'95 have both....still OBDI under the hood which you can use...and OBDII in the center console.
In '95 Volvo 850 introduced OBDII but still have OBDI under the hood.
'96 and '97 have OBDII only.

dnd2526 · Post by **dnd2526** » 11 May 2020, 16:04

Does the OBDII port provide complete access to any stored codes? I've used my Bluetooth scanner to monitor various sensor data also but I've read that they are not very accurate and I don't really know what to look for other than inconsistencies in the data. I watched the video for testing the relay. The relay looks very similar to the one that controls the headlights on my '95 Saab 900 which is also notorious for failing due to cracked solder joints. I successfully repaired mine and may inspect the Volvo relay for similar cracking and repair it if necessary.

Post by **misha** » 11 May 2020, 18:12

Yes it does....but it also depends on the scanner/scan tool.
Fuel pump relay is not like a headlight relay...it have some capacitors in it which can fail in addition to cold solders.

dnd2526 · Post by **dnd2526** » 12 Jun 2020, 05:46

Finally got around to checking the codes. I have also replaced the throttle position sensor since I had one on hand. It hasn't been as bad since then but I can still feel some hesitation and sagging under light throttle and almost always a moment after the transmission upshifts.

Here are the codes:

232 Long term fuel trim, idling
143 Front knock sensor signal <- I have heard the engine pinging when it is running poorly, particularly on hot days.
315 EVAP system

Thanks for the help.