hesitation when accelerating...

[ryank]

Car is a 94 940T. Engine is stock except for MBC and RSI cam. When I accelerate just after starting the car cold, it feels as if I'm lifting the gas every few seconds, like it pulses. To see if this was an under-load-only situation, I tried holding my foot steady on the gas while in park. As you can see from the video, the rpm are fluctuating, my foot is not moving. In the video, I let it drop back to idle and then tried again, pressed the gas and held it steady, and the rpms still fluctuated.

Once the car sees about 30-60 seconds of driving, or I start with a warm engine, the problem nearly disappears. However, if I'm going steadily up a hill, I can see the boost fluctuating in the same maner, it's just very hard to perceive it.

I've replaced plugs, cap, rotor, wires, and FPR. Pulled the fuse for the in-tank pump at idle, and can definitely hear the in-line pump straining, so I know they're both working. Rather than throwing money at parts that may be fine, where should I go next? Thanks. Oh, and no error codes from the OBD.

http://youtu.be/-f79qEuf0sY

[ryank]

any thoughts?

[740tankDriver]

sounds like ignition timing is moving around by the computer. or it could be something else. how is your knocksensor? on too tight?

[Canuckvolvo]

My guess is your cold compression is a bit off, but evens out as everything warms up. At least that's how my 940 is. I would suggest cold and hot compression test- if I had done that when I was trying to diagnose the same problem 5 years ago I would have found a bad head gasket alot sooner....

[ryank]

Originally Posted by 740tankDriver

sounds like ignition timing is moving around by the computer. or it could be something else. how is your knocksensor? on too tight?

I just pulled knock sensor, cleaned the already clean contacts, and retorqued, no change. Also no codes which I believe a faulty knock sensor would throw.

Compression check, I've not done, so probably worthwhile even to establish a base line. What numbers should I be looking for on this engine?

[TrickMick]

Originally Posted by ryank

I just pulled knock sensor, cleaned the already clean contacts, and retorqued, no change. Also no codes which I believe a faulty knock sensor would throw.

Compression check, I've not done, so probably worthwhile even to establish a base line. What numbers should I be looking for on this engine?

ROT...15% of the highest number - across the board
it's not necessarily the "numbers" - rather the range..(unless you
are showing 95 lbs/in on all four...)*THEN* ya got trouble....

1) booster battery hooked up
2) throttle jacked wide open
3) all plugs out
4) same number of *chuffs* for each hole tested

i do them cold so I can see if there is a "sealing problem...

WET test the same way except you drop a tablespoon of
engine oil down each plug hole just before you spin it....'

record results and post back...

[ellist]

I'd also check the fuel injection seals. They may be swelling after the car warms up. Fill a spray bottle with water and mist around the fuel injectors when the car is cold and running. If it stumbles/dies replace them all. It's cheap and easy to do. Also, how long has the fuel pressure regulator been on the car? Do you notice the smell of gasoline when starting cold and or a slight puff of smoke when starting cold or hot? Those can indicate the regulator too.

[ryank]

Thanks for your help guys, reporting back with some info.

Sprayed the injector seals, and it may have run a little rougher, but it was pretty subtle. I found a set cheap so will replace them anyway just to rule that out.

Compression numbers are as follows:
Cold
1: 120 psi
2: 110 psi
3: 130 psi
4: 130 psi

Hot
1: 120 psi (wet +0 psi)
2: 110 psi (wet +2 psi)
3: 125 psi (wet +10 psi)
4: 125 psi (wet +10 psi)

Also took shots of the plug while they were out. These are BPR7ES with 2k miles on them (at same service I cleaned the throttle body, replace intake manifold gasket, replaced FPR, replaced the PCV breather box and cleaned the PCV lines.)

[TrickMick]

Originally Posted by ryank

Thanks for your help guys, reporting back with some info.
Sprayed the injector seals, and it may have run a little rougher, but it was pretty subtle. I found a set cheap so will replace them anyway just to rule that out.
Compression numbers are as follows:
Cold
1: 120 psi
2: 110 psi
3: 130 psi
4: 130 psi

Hot
1: 120 psi (wet +0 psi)
2: 110 psi (wet +2 psi)
3: 125 psi (wet +10 psi)
4: 125 psi (wet +10 psi)

Also took shots of the plug while they were out. These are BPR7ES with 2k miles on them (at same service I cleaned the throttle body, replace intake manifold gasket, replaced FPR, replaced the PCV breather box and cleaned the PCV lines.)

1) compression acceptable (the engine *IS* getting tired but it *should still run
well enough to get you about*)....
2) #4 cylinder is showing "fat" compared to the other cylinders...when you go to replace
the injector seals cycle them and check the SPRAY PATTERN...if it were MY car,
I'd get them clean and flow tested.....
3) check your O2 sensor for SWEEP and back probe the AMM and check it's signal...
(as always....thorough checking for VAC leaks *and* do the Volvo lektrikal mantra:

repeat the VOLVO LEKTRIKAL MANTRA with me *ONE MORE TIME* !
1) clean / tight /correctly sized CONNECTIONS *EVERYWHERE*
2) look for "creeping blue/green CRUD *up under* insulation" on BATTERY CABLES
3) fuses scrubbed and cleaned "contacts bent SNUG (if porcelain type use the COPPER
coloured ones *NOT* the "tin ones")
4) alternator terminals clean snug and GROUNDED to the BLOCK side of the motor mount and safe to the alternator
5) repair / replace harness bits that are crumbly-insulated on the "notorious DEATH PERIOD for harnesses"
6) when in doubt VOLTAGE DROP TEST / check RESISTANCE ACROSS a CONNECTION
7) dielectric grease is an insulator..and that's as far as I'm gonna speak on the subject..I *DON'T* use it
on ANY electrical connection...I DO *seal* the back sides of connections to stop gas intrusion with it....EOS

once you've got the "wahrs sorted" you may proceed to testing VOLTAGE /
RESISTANCE / CAPACITANCE / INDUCTANCE / ETC....

the "notchy feeling" (as I read your description) sounds like the FUEL MAP is getting
abused by signal inputs....thus the call for O2 sweep and AMM measurements...
also check the THROTTLE SWITCH for correct op and the CPS for *STEADY* output
(if the CPS is funky or sending "random signals" it will cause the "computerS" to fuss!
let us know how it goes....

[ryank]

I'm totally in agreement with your wiring mantra, I routinely inspect and clean connectors just to make sure they're in good working order. The wiring in my car is very clean, no corrosion, tight grounds, no brittle wires, etc. I had some time to run a few tests , but also ran into some questions...

O2
I have the type with all three wires going into a single, sealed, orange connector on the fire wall. How do I test that one with no way to back probe the sealed terminals? can I test at ecu?

CPS
Was able to get near new ohm reading with engine still. Didn't have helper to crank. Which of three terminals am I checking when cranking?

ECT
Got approx correct voltage measured at ecu.

AMM
Ground to sensor checked out good. When plugged in, could not test voltage to terminal 3. Can run the same test to some pin at the ecu?

TPS
Has the click correctly set, but could not get in a good position to see the terminals to test. Will ahve to try again with a mirror.

Other side notes...
Found a loose intake hose to turbo inlet connection. I check because I noticed my boost gauge running less vacuum than normal at idle. I also checked valve clearance because I thought I heard a little noise at idle. All were between .017 and .020, so probably just the slight exhaust leak I have at the turbo>downpipe.

Im starting to think the O2 sensor is probable concern. My cat's been rattling since I got the car, and the exhaust is woefully patched together likely causing a lot of back pressure. I've been planning on doing a new turbo-back and O2 sensor when I change to a 15g, just haven't made much progress on that. My mileage is also suffering which could be a result of both clogged cat and O2.

[ryank]

Update. Left the #1 fuse (engine control, fuel pump) out overnight. Plugged it in today, started the car, and pulling out on to the road for the first 20 seconds or so, NO hesitation! After the 20 seconds I felt it again. It's always most pronounced when I first move the car after a cold start, so I was pleasantly surprised. I think this also confirms an electronic issue.

[ryank]

Another update; Friday the car quit while on the highway (in traffic.) Second time, so I had to investigate. Yesterday I checked the fuel pump relay. The ignition on/off worked fine, so I jumped the pins to allow continuous operation. Both pumps operating. I decided to open the fuse up anyway just to make sure there weren't any bad solder joints, etc. It was pristine inside. I then checked the RSR. Same deal, pristine. Lastly I thought of maybe pulling the crank position sensor to see what the end looked like. When I removed it, I saw a couple wraps of electrical tape combined with completely missing parts of sheathing down by the sensor itself. It was heavily coated in oil, and the end of the sensor was black and crumblyit . Given the condition alone, I'll replace and won't be surprised if that solves the problem.

[TrickMick]

Originally Posted by ryank

Another update; Friday the car quit while on the highway (in traffic.) Second time, so I had to investigate. Yesterday I checked the fuel pump relay. The ignition on/off worked fine, so I jumped the pins to allow continuous operation. Both pumps operating. I decided to open the fuse up anyway just to make sure there weren't any bad solder joints, etc. It was pristine inside. I then checked the RSR. Same deal, pristine. Lastly I thought of maybe pulling the crank position sensor to see what the end looked like. When I removed it, I saw a couple wraps of electrical tape combined with completely missing parts of sheathing down by the sensor itself. It was heavily coated in oil, and the end of the sensor was black and crumblyit . Given the condition alone, I'll replace and won't be surprised if that solves the problem.

good catch....the "bottom end" will get moisture in it and start to corrode ... when THAT
process starts, the "magnetic bit" will get disrupted and the signal goes to hell in a
hand-basket tuit de suite !
let us know how it shakes out!

[ryank]

New CPS installed, and no dice. The car does seem to idle a little bit smoother, but I'm still getting that pulsing acceleration when cold. So far it hasn't died on me again, so hopefully the new CPS has solved for that.

I ended up ordering a O2 sensor, so I think I will put that in and what that does...

[ryank]

New o2 sensor definitely helped. I think it was compounding the hesitation problem as I can still feel it pulse, but not as severely as before. The car seems to be a touch more responsive as well. The old sensor was fairly heavily covered.