Low RPM hesitation

[Lukas]

Just a bit of background on 4322 first - I bought the car with about ~2000 miles, and have since put on another ~1000 in 1 year since. The car has sat for long periods, and previous service records indicate fuel system issues. The only major modification is a twin muffler/header setup with no crossover.

The initial issue was a hesitation at HIGH RPM and more than 1/4 throttle. The car did not want to rev much over 4000, and felt like it had no power. There was also a change in the exhaust tone when it did this - started to sound like a Subaru exhaust. From idle to about 2500-3000 was fine, regardless of throttle. Idle was steady, and there were no cold/warm/hot starting problems.

Both myself and another D owner have been working on the car (mainly the other owner, who has a significant mechanical background)

The parts below were replaced, but did not solve the issue:
-Fuel pump (new combo unit)
- Spark plugs and wires, electrical distributor
- Coil
- Fuel filter
- Air filter
- O2 sensor

After an early morning phone call to Dave at DMCMW, we pulled the injectors - passenger side bank was just leaking out and fouling the plugs on that side. Driver side bank ok.

The injectors and seals were replaced (spray patterns were good before going in), but now a new set of symptoms have occurred:
- Low RPM hesitation with change in exhaust tone to what was described above. I think it is throttle dependent, but need to do some more driving to confirm.
- Hesitation seems worse when warm, occurring at lower revs.
- Runs very well once above about 2500RPM, pulls strongly to redline and normal exhaust tone
- Idle less steady
- May stall coming up to traffic lights, when clutch is depressed - never happened before
- Disconnecting the FV seems to help, but again I need to do more driving to check
- Cold/hot starts still fine

Basically the symptoms have flipped from high RPM hesitation to low RPM hesitation, upon change of injectors.

Based on my readings, I think we're now looking at checking the CPR (check to see if screen is clogged) or fuel distributor (sticky plunger).

We're fortunate to have Dave Swingle in Australia, and visiting my city on Sunday for a tech session and lunch. Hopefully I can get him to drive the car and see what he says, but before he arrives, I was wondering whether any forum members would have some ideas I could discuss with him.

[Bitsyncmaster]

Have you adjusted the mixture yet? Sure sounds like a mixture problem. I would start with checking fuel pressures.

[DMCMW Dave]

Yeah - if you can round up a fuel pressure gauge and a dwell meter it would help, but I'm sure we can get close.

[hmcelraft]

Have you verified that the vacuum lines are correct and the vacuum delay valve is in installed in the correct direction? The "Dist" (S/B white) side of the delay valve should be on the vacuum supply side - although - if the engine is warm this valve is not relevant - IF - the vacuum lines are connected properly.

[Lukas]

Hi guys

Thanks for the responses so far.

Vacuum lines have been checked and apparently are all ok. Will double check at the tech session

We've been playing around with the mixture, but haven't used a dwell meter to set it yet - we'll bring one along to the session.

[Lukas]

I was fortunate enough to have Dave Swingle look over the car, at our tech session today.
- Timing was off - fixed.
- Couple of other vacuum leaks - fixed
- Mixture adjusted with a dwell meter
- FV confirmed to be working, but it does swing around a lot.
- we did not have the equipment to test fuel pressures, but Dave thinks this is unlikely based on his test drive

Good news is that idle is steadier, and the car no longer feels like it will stall when pressing the clutch. I also hit some traffic on the way home, and the car was running quite well/cool. It's significantly more driveable now.

However the low RPM hesitation is still occurring! - basically no change to that.

Based on his work with the car today, Dave thinks it's unlikely to be fuel distributor or CPR now. He has left us with some ideas - we're now looking at checking/changing the spark plugs (and associated wires/coil - although new parts, they could be bad), and also hunting down any air leaks - the pipe of agony will need to be sealed better (just has some electrical tape currently) - will post an update when this is done.

Any other ideas?

[Bitsyncmaster]

I was fortunate enough to have Dave Swingle look over the car, at our tech session today.
- Timing was off - fixed.
- Couple of other vacuum leaks - fixed
- Mixture adjusted with a dwell meter
- FV confirmed to be working, but it does swing around a lot.
- we did not have the equipment to test fuel pressures, but Dave thinks this is unlikely based on his test drive

Good news is that idle is steadier, and the car no longer feels like it will stall when pressing the clutch. I also hit some traffic on the way home, and the car was running quite well/cool. It's significantly more driveable now.

However the low RPM hesitation is still occurring! - basically no change to that.

Based on his work with the car today, Dave thinks it's unlikely to be fuel distributor or CPR now. He has left us with some ideas - we're now looking at checking/changing the spark plugs (and associated wires/coil - although new parts, they could be bad), and also hunting down any air leaks - the pipe of agony will need to be sealed better (just has some electrical tape currently) - will post an update when this is done.

Any other ideas?

Have you replaced the O2 sensor yet? Hesitation problems are 90% fuel mixture problems.

Also check that your vacuum advance is working. (not on at idle and on off idle)

[Spittybug]

I'd really check the vacuum advance system; microswitch, solenoid, hoses & distributor unit. If it isn't functioning, you step on the gas, RPMs go up but they don't advance the distributor until ~1,500-2,000 RPM. You need the vacuum advance (at its maximum of ~20* advance right as you come off idle) to send the spark MUCH earlier than the static ~15*. By the time your RPMs are kicking in the mechanical advance, your vacuum advance is tapering off because you don't have as much vacuum being generated in the engine because the throttle is open letting in lots of air.

You can test the hose and distributor unit just by sucking on the hose and watching to see if the plate rotates (cap off of course). To test the rest, simply disconnect the hose from the distributor side; at idle you should feel no suction since the microswitch is keeping the solenoid closed. Off idle you should feel vacuum on that hose.... if not, work your way backwards. Vacuum solenoid connected? Vacuum hoses in valley of death connected to thermal vacuum switch in y pipe, vacuum switch hosed to manifold, throttle microswitch working?

Good luck.

[Lukas]

Thanks again for the responses.

Some of the D owners are having dinner with Dave and Julee tonight, so I will show him the responses. I believe he's off doing some tourist activities today!

Will check the O2 sensor next weekend - I've heard they can be fouled by just a few minutes of the mixture being off.

[Soundkillr]

Make sure your warm up regulator does not have vacuum once the car is hot. My solenoid went bad and always allowed vacuum on the wur. It would be fine cold, but once hot the car would load up and hesitate like crazy on low rpm.
Wur should have vacuum when cold, but none when hot.....

[Lukas]

Alright, time for an update on this.

A previous post mentioned the vacuum advance - Dave told me that this was ok at the tech day

Things we worked out today:
- Upon firing up the car after sitting for 5 days, there was no hesitation through the RPM range and the car drove like a normal one should. Once the car had warmed up, there was once again low RPM hesitation.
- Pulled all spark plugs to check - all ok except for driver side rear plug was black
- Replaced all spark plugs - no difference to hesitation
- Decided to change the injector associated with the above spark plug. This was a new injector from DMCMW. It was swapped with a spare original injector. This altered the symptoms, so that I was now experiencing the original problem (high range RPM hesitation) but nowhere near as bad. Swapped back to the new injector and back to low RPM hesitation.

Unfortunately I don't have access to fuel pressure gauges yet - looking at getting a set.

Is it possible that one of the new injectors is bad? Dave sent me 6 new ones - actually one of them did not spray. So the car has 5 new injectors, and 1 old/original injector. Before the new set went in, spray patterns checked out ok, but we did not check actual flow rates.

Alternatively, what about a bad (aftermarket) electrical distributor?

[Soundkillr]

I know you said vacuum advance was fine, but please make sure to double check that there is no vacuum at the warm up regulator once the car is hot. What you are describing is the exat symptom my car had when the thermal valve want bad. It allowed vacuum on the warm up regulator and caused the car to load up with fuel at idle, and it would hesitate at low rpm and idle.

[Lukas]

I know you said vacuum advance was fine, but please make sure to double check that there is no vacuum at the warm up regulator once the car is hot. What you are describing is the exat symptom my car had when the thermal valve want bad. It allowed vacuum on the warm up regulator and caused the car to load up with fuel at idle, and it would hesitate at low rpm and idle.

On the vacuum hose diagram, was that the solenoid valve, or the thermal control valve under the intake?

[DMCMW Dave]

On the vacuum hose diagram, was that the solenoid valve, or the thermal control valve under the intake?

That would have been the thermal control valve under the intake. This could be checked with a vacuum guage for sure, but we checked it for vacuum with my finger when I was there and it seemed OK.

I know the timing side is working as we verified it both via vacuum and the timing light.

You may be dealing with an injector problem. I have seen cars that didn't have the system complely flushed, and I mean completely, plug brand new injectors. If there was some spare crud in the fuel distributor or fuel lines it can easily break looks once you start driving the car and plug up a new injector or two.

The car runs (on purpose) much richer when cold (this is the purpose of the warmup regulator), so a marginal injector that is opening "late" may not be as apparent. I have a suspicion you are trading an new injector that isn't flowing at low pressure for an old injector that isn't flowing at high pressure. A full all-injector flow test would be my next step, as this would also make it easy to diagnose something amiss in the fuel distributor.

Since you need to pull the center section to fix that idle speed pipe anyway (that does need to be fixed as it is still likely a low speed unmetered air leak) it would be very easy at the same time to pull and test all the injectors.

[David T]

With all of the things you did, now you have to check more basic things. To get the motor to run smoothly, especially when going slowly, all of the cylinders must be balanced and operating as close to the same as possible. This means checking compression and valve adjustment. It also means checking each injector and the flow out of the fuel distibuter. On one car that had a bad idle I found that that someone had replaced the impulse coil. That wasn't so bad but the reluctor was bent up and not equally gapped all around causing each cylinder to fire erratically. I am not saying this is your problem but now everything must be checked and double checked till you find the cause. This is the kind of problem that causes most mechanics to throw their hands up and walk away. It is also very distressing for the customer because he could be spending a lot of money for the mechanic's time with no results unless they both get lucky quick. The very worst approach is to "shotgun" it by throwing parts at it. You will be replacing a lot of good parts. This HAS to be approached in a methodical style with some kind of plan.
David Teitelbaum

[Lukas]

Thanks, will run those tests and see what happens.

[Lukas]

Haven't had much time to work on the car, but ran a quick test with a new injector in the spot that had a fouled plug, and another new injector to replace a 30 yr old one still in the system.

Hesitation is pretty much gone - I'd say 95% - not quite perfect but the car is running at it's best since I've owned it. As an unexpected benefit, I no longer have idle hunting on cold start. Next to pull all injectors to test, fix the air leak and check CPR pressure with the gauges which should hopefully fix any remaining issues.

Interestingly, I was sent different part #'s for the injectors. I am basically running two of the 0 437 502 022 (this matches the original injectors I pulled out) and four of the 0 437 502 013 (this part number appears on some crossref lists as well). I believe the 022 were used in the Volvo 760 with the same engine (at least in Australia according to a Bosch parts book).

Could mixing different injector #'s cause the above behaviour (very minimal hesitation)? Different spray patterns and opening/closing pressures?