740T slight stumbling and hesitation under acceleration
 

Hi all. I am new to the forum and slightly new to volvos. My dad drives a '86 240 wagon which he bought brand new and has put 378000 miles on it. The car is 100% stock and we are both proud of it. Anyway I am a college student who knows a little about cars and bought myself an '87 740 turbo wagon. The car is awesome. Bought it for 2100 and put about 1500 in aesthetics, AC and a rebuilt overdrive.

The car has had many hours of TLC to get it back to tip top shape. I have all the bugs worked out except a slight hesitation under moderate acceleration.

I will explain the symptoms as best as I can and then state all the routes I have tried to fix the problem.

This stumble/hesitation happens between 1800 and 3000 rpm under very moderate acceleration. Full throttle the car is very smooth. I know that it is a lean condition because the plugs have a charred white color.

After removing the stock plugs I replaced them and found that after a few weeks the new plugs appeared the same.

The next thing I checked was for vacuum leaks. using propane, carb cleaner, and a hose to listen for the leak. No luck.

So did some research on what the AMM is and what it does. Grabbed some amm cleaner from kragen and cleaned the sucker and stuck it back in. No change.

I bought a new wax thermostat for the air box thinking it was sucking in hot air but the inexpensive part wasnt the cure.

Next I went to pick and pull and for forty dollars grabbed another AMM off another turbo and put it in. the car still performed exactly the same. I thought that it would give me a slight change but no.

I changed the 02 sensor as well but nothing. Checked the fuel pressure and it is good. I removed the vaccuum line off the fuel return valve and it is working correctly because the pressure increased. Checked the throttle body and it is clean. Car has 168000 on it and was 100% stock when i bought it. I have not made any motor modifications.

Because it only does it under a very light load I think it must be some electronic sensor that is telling the car to run lean and I have done everything in my power to fix it I am stumped.

I took it in to my local euro shop that my father has done business with for many years and the said it is the car is set up to run lean and the stumble is the way it is. I trust their knowledge but feel that they just told me some B.S. so that they wouldnt have to hunt down the problem.

Im getting tired of throwing parts at the car. It must be a very small possibility that the used mass air meter was also slightly defective? I dont want to buy a brand new one for 300 bucks if that is not the problem.

If anyone has ran into something like this or has any ideas I am all ears. Thanks

You're right; they fed you some BS! And...probably because they didn't want to chase it down...and, because you're young. I'm an old fart now but I still remember being young and folks not taking me seriously.
Your 87 has LH2.2 Jetronic fuel injection. It has a Hall sensor on the distributor, the wiring may be bad. Don't buy anymore parts. The O2 sensor is a good thing to have replaced. I will tell you that w/ my 84 760T, I had a very similar issue. And did similar things to hunt it down. I ended up doing a couple of things to resolve it. I replaced EVERY vacuum hose, including the idle air control and brake booster lines...all of them. On a turbo, the vac lines do double duty as they also carry positive pressure when under boost. Check and see...I know you said moderate throttle, but see if the boost gauge is positive or not when this occurs. Make certain every vac line has a clamp at either end. Pull the vac line off the fuel pressure regulator while the engine is running. The idle should change...if not, smell the end of the vac hose...if it smells strongly of gas, the diaphragm is bad (normal to smell faintly of gas). Smell the dipstick...if it smells of gasoline, the fpr is definitely bad. Pull the throttle body and clean it thoroughly. It's impossible to get it spotlessly clean attached to the car. It is definitely not normal for a turbo car to run lean, in fact it isn't desirable. They typically run rich to pig rich. That's to keep the piston crowns cool and preclude detonation.
There is another thing to check too. LH2.2 uses the 007 AMM which is adjustable. Check it out in a manual or slide over to turbobricks.com as I remember reading an article about using resistors and an LED to dial the AMM in correctly. Also, take a very close look at the female connectors in the AMM plug...heavy handed wannabe's often bend the connectors so they no longer make contact; too, there could be corrosion.
The O2 sensor should tell the ecu that the mixture is lean and the ecu should compensate by adding more fuel. The fact it isn't means it is receiving bunk information...or it is fubar'd and can't because it's toast. Under wide open throttle (WOT) the ecu reverts to pre-programmed fuel maps and does not use the sensor input...it just throws a boat load of fuel at the intake! Your car came from the factory w/ a 541 ecu. When they failed, the dealer would install the slightly smarter 591. The latter is a better ecu, imho, but the 541's are plentiful. $30-40 bucks on turbobricks. After you check all the other stuff...if it's still missing, swap in a known good ecu.

I go the same problem.....86 240 wagon...the bogging happens a the same rpm and when I turn right..weird....just right, left turns are smooth...

replaced the fpr with a new one
IAC valve
booster check valve
ac valve
fuel pump, intank & outer

Redbottle did you replace the sock? How about the fuel filter?

I just had a very similar problem with my 89 740T right after I replaced a flakey Bosch Power Stage with two different aftermarket replacements (KAE, Hella). Then I picked up an OEM Bosch (made in USA) Power Stage, and my acceleration hesitation disappeared. Check your Power Stage and make sure it's a Bosch. Grab one or two from the junk yard. The Bosch unit costs between $100-$200 new.

if you have not, also read up in the "700/900 FAQ"

Put a multimeter on the airmass meter and the O2 sensor testing plugs...make sure you have a proper "sweep" from both sides and is within the spec settings. This way you can see if you are running too rich or too lean or just right. Hence you can see for certain if either the O2 sensor is bad or the MAF needs adjustment or is bad or if it's something else...just cleaning the MAF will not do a thing...you need to TEST it. Once the platinum goes bad..it's bad and no amount of cleaning will help it...Also, your problem could be electrical but you need to start testing first and cross off what it's niot instead of just throwing parts at the problem...hope this helps and good luck!!

Thanks everyone. I will make a list of things to start checking again from the previous posts and hopefully I can hunt down this gremlin...I was thinking about the new ECU and have been looking at them on the internet. I am pretty certain the vacuum lines are solid and do not have any leaks but replacing every single one is a good idea....I just don't know if the stuff from kragen is worse than the 20 yr old lines on the car to begin with so i was going to go through the dealer for the lines.

As for the adjustment on the AMM i have fooled with it plenty. I set my multimeter on the 02 sensor and adjusted the small screw in the front and watched the volts change. I know it should be in the range of .1 -.9 volts. I was setting it up rich at idle but a mechanic told me that this was worse for the lean situation because it was idling rich the ECU was responding but leaning out the mixture as the rpms increased...another buddy of mine told me that the adjustment screw on the AMM is specifically for idle and would not be a source of my problem since the car idles fine.

swiftjustice, I checked my boost and the problem occurs with both positive and negative pressure almost identically...Although with the engine cold the stumbling is very noticable with positive boost but once it warms up it gets better.

I drove my dads naturally aspirated 240 wagon today...although the torque and hp are not as abundant as my 740T the car is much more smoother and a joy to drive.. with twice the miles the 240 seems like a more solid vehicle. Both cars have manual trans... making it easy to feel and observe the issue.

'86 245 w/ 4speed and overdrive- 379k
'87 745T w/ 4 speed and OD- 169k

Running lean can also often be caused by a flaky coolant sensor. I'd also check ignition timing, specifically use a timing light to look at the ignition advance as you rev the engine slowly through the problem rpms.
However, I cured a myriad of small flat spots and light stumbles on my LH 2.4 turbo by replacing the O2 sensor. They do have a service life, and it's about 60k miles (100k kms).
They should be treated as service items, replacing them every timing belt change is good policy.

Regards, Andrew.

Alright guys... I think I am getting closer to the problem but then again I feel like Im running around in circles.

I put a timing light on the car and you can easily see the problem by watching the index on the pulley. The car idles smooth and as you turn the throttle the ignition will advance and then the trouble starts... The timing mark bounces back and forth about ten ten degrees just as the engine surges.. it will act up around 2000 to 3500 rpm and after that it smooths back out.

As I have been doing my checklist is growing smaller. I know think it is a strictly ignition problem and have replaced the following:

power stage, hall sensor on the back of the distributor, and two different knock sensors with spares from the junkyard and the car runs and acts EXACTLY the same. Not a change in sight. I unplugged the temp sensor that is mounted to the block while the car was running... checked the timing and it still bounces... all that happened was the temp gauge inside the car reads zero.

soooo is the temp gauge that is mounted onto the radiator the one that is connected to the ecu?

...from what I have read all things point to the knock sensor and for whatever reason it is retarding the timing as if the engine were knocking...but it is not. So I unplugged the KS while the engine was running and it did not change and the motor did not care whether or not it was plugged or unplugged.

Im all ears to any more ideas and this issue is starting to get under my skin... and into my wallet:(

It's an ignition problem, let's concentrate on that. I would start looking at The Hall sensor again, ensure it has the correct gap to the reluctor (toothed wheel). I once had a reluctor go bad on an engine, had no magnetism. It would play up and throw timing all over the place too, when it started.
Also check the rotor is a nice tight fit on the shaft, no end float or rotation back and forth.
It could also be a distributor shaft wear issue. As the 87's use the distributor for spark distribution and sending the ECU a TDC signal, I'd be inclined to pull the distributor off and really go over it thoroughly. I think you have a bad connection somewhere, either in or close to the distributor.
Easy way to tell if an ECU is responding to a knock sensor is to unscrew it, leave it plugged in, set up timing light and tap the knock sensor sharply on the block repeatedly, it should retard timing, but I don't think it's your problem.
Just had another idea, check your timing belt is tight. I had a Nissan that would send a resonance through the belt at 1400 rpm and the spark would jump around like this. If the timing belt is loose, teh cam lobes can set up a resonance in the belt and cam.

Regards, Andrew.

The temp sensor that sends signals to the FI ECU and the Ignition ECU is located under the intake manifold, 3rd runner back from the front of the engine. You basically can't see it, and it's a royal pain to get a-hold of without taking things apart. The sensor has is actually a two-in-one, one for the FI ECU and the Ignition ECU The Sensor that you can see, and were able to disconnect, is only for the instrument cluster. The one in the radiator is only for the supplemental electric fan in-front if of the AC condenser.

Also Typhoon is mixing up crank shaft position senors. A hall sensor has it's own internal magnet, and the "toothed" (4 gaps) metal wheel is not in and of it's self magnetic. There is a type of position sensor that uses a magnetic "tone ring" and coil to generate a signal, but it's not used on the 88 B230FT with EZK-117, which is what you should have. You should do a google search for TP31397, one of the few volvo greenbooks that can fairly easily be found on the internet, and just happens to cover your ignition system. Page 20 has some nice pictures and explanation of the Hall sensor and how it essentially works.

Oh, I'm not mixing anything up. I'm fully aware of how Hall sensors work in automotive applications, I've been working with them since the mid 80's, how about you?
Any Hall sensor relies on the principle of a magnetic field being generated and collapsing to generate a signal. Guess what? Anything made of iron is magnet to varying degrees. You do know that a "magnetic" tone ring and coil pickup is a Hall sensor, don't you?
Does the 740 being discussed here not have a Hall sensor in the distributor to send a timing signal to the ECU? If so, everything I said is completely relevant. All you've provided is irrelevant minutia that will only confuse the OP and lead the discussion away from a solution, in fact, I don't see any real solution offered by you.

Regards, Andrew.

Sorry Typhoon, I wasn't trying to suggest that you didn't know how the system works, I just thought you were describing parts of two different systems. I'm using Volvo GreenBook TP 31397/1, page 20, as my point of reference for Hall Sensors. So if we agree on what it says there, then we agree. Which I think in reality we do. Also I'm sure you have more automotive experience than I do, but I have been working on volvos since the mid 80s. Between us and everyone else I'm sure we can help Will1147 with his current repair challenge.

TP 31397/1, look at p20, picture B is slightly mangled unfortunately.
http://oak.cats.ohiou.edu/~ridgely/V...nition-OCR.pdf

I did change the fuel pump & filter. I tried putting back the old one and the engine ran like shiskabob....I was not able to buy a good fuel filter and ended up with a CHEAP, Quazimoto for 8 bucks...

I agree with the guys here about the coolant temperature sensor....
mine was from a junk yard, and so was the fuel pump.....my intank pump is new.

What is a sock?

I have not replaced the coolant temp sensor yet because it is a bitch to get to and after some research I do not feel it is my problem...correct me if I am wrong but the temp sensor only advances the timing when the motor is running hot and the throttle postion switch is closed...in doing so it tries to keep the motor from overheating. So if my timing was bouncing at idle (TPS closed) then this would sound like a faulty temp sensor but because it only changes the timing at idle and when the engine is overheating I feel I can check this off my list of problems.

Back to the knock sensor...I was reading that if torqued incorrectly the sensor can give false information (false knocks) to the ECU and throw off my timing. Sooo with the engine warm I unbolted the sensor and left it plugged in. No change in timing...it still does not sit still, but retards.

The hall sensor was completely replaced by one from the junkyard that was basically brand new. It looked brand new and on the back of the cap it said re manufactured so I know it was not stock but a newer unit but still bosch with the correct part number. This new hall sensor had absolutely no shaft play and the toothed wheel looked very clean as well as the magnet with no corrosion or anything odd or out of place...The hall sensor that came out of my car looked a little rougher and the seals looked tired and needed replacing but the most significant find was the shaft play was horrible...I was set that I had found my problem!...that when operating the car the shaft was moving back and forth messing with the sensor pickup...but after installation of the new one...there was still no change on the behavior of the motor....what a surprise...

I have stared at the timing mark miss behave so much that I will repeat more in depth of what is going on so hopefully we can center in on the problem easier.

When the engine is cold the bogging/hesitation behind the wheel is about three times worse and noticeable than with the engine at normal operating temperature. I have not put a timing light on it when the motor is cold to see how far the timing is bouncing but I can state what it is doing when the engine it hot....at idle the mark sits at the usual 12 degrees above TDC. when the throttle is increased the ignition timing will advance like it should normally but every half a second or so it will retard about 5 or 10 degrees. Once higher RPMS are reached (4000+) the timing will be perfectly smooth.

To my dismay and frustration the car is going back to the shop in two days under an ignition issue rather than a lean condition.

Again, thank you everyone for your help. Your ideas and advice are much appreciated.

Best of luck Will147, I hope the shop finds the problem quick. Please let us know what the solution turns out to be!

I agree a faulty computer temp sensor will make car stumble but usually only at first go and then it will clear up as car heats up...

Well just got back from the shop...no luck. again this is the second time the shop has looked at the car and now that they know the ignition is out of wack they have less of an area to diagnose...but still they found nothing.

We both know that something is telling the computer to retard the timing but because all the sensors that were suspicious were replaced, the only thing left was the ignition control module. soooo I ran to the junkyard and grabbed a replacement and plugged it inwhile the car was undone at the shop crossing my fingers. No luck.

While under the hood the mechanic noted that with the AMM unplugged, AKA in 'limp home mode' the timing mark stays steady. Because of this he feels that the main engine control unit may be faulty because the AMM sends a signal to the ECU which then sends a signal to the ignition control unit saying whats going on.

All in all this issue becoming pain in the ass. So tomorrow I will be back at the junkyard swapping my ignition control unit for the main ECU.

P.S. while I was looking at my ECU in the car under the kick panel I pulled off the main connection. While doing so I noticed, coming from the connection that plugs into the ECU, all the wires run forward into the car or elsewhere except for two. These two wires come straight out from the heavy black tape that is bundling together and the wires go into a standard black connector. The male end holds the two wires out of the wiring bundle and the female connector has a red wire that has some shrinkwrap around it that both prongs inside the connector are hooked too. basically creating what seems to be a closed circuit. This just seemed odd to me and I will look tomorrow at the donor car to see if it is the same. Or If someone has been in here and tampered with something...

Thanks Again everyone.

Oh and just for ****s I am going to grab another temp sensor just to cross one more thing off the list of possible items.

Used ECU and a brand new temp sensor did not help. What else would retard the timing in this faulty manner? Like I said before with the AMM unplugged it reverts back to the limp home mode mapping. Because of this the timing remains steady. I think this may be a clue to what I am looking for. My spare parts pile grows larger everyday it seems like. The shop is baffled so I might try one in Sacramento next week and see what they think.