Hi all,
Could do with some help to resolve the latest gremlin to afflict my 88 V12, 5spd, 150k km.

1,000km of trouble free motoring since work done over the winter by my mechanic. Two weeks back I did a 400km day trip and experienced the following:

1. Engine faltered ('jerk') while accelerating at 2,000rpm in 4th.
2. Upon restart after 200km driving and sitting for 3hrs the coolant light stayed on for 20 sec (coolant level fine).
3. Near end of return trip in heavy start-stop traffic, 27 degC air temp, the engine died with no warning. 10 mins later restarted fine. This was repeated 10mins later, at which point it got towed.

The PO replaced the auxiliary air control valve with a simple 'screw in, screw out' valve to regulate idle speed. This was done to avoid high start up rpm. Now there is no 'choke', so needs extra gas to start up, but seems to work.

My mechanic determined that the idle screw had loosened resulting in low idle, too rich a mixture, hence stalling. He corrected this, reset idle and I picked it up today.

Brutal traffic on way home and engine died twice, as before. Bit nerve wracking, as 2nd time stranded in middle of busy intersection! Both times I think I heard a 'whomping' sound from rear of car, but a lot was going on (pressure release from gas tank??).
Just spoke to my mechanic and he suggested replicate stalling under safe conditions and remove / replace gas cap to see if it resolves it (could be as simple as faulty gas cap). Engine also faltered at 2,000rpm on hwy for a split second while cruising with foot lightly on gas ( may be unrelated).

Grateful for any and all thoughts!

Quentin

 

Was the ECU modified to account for the 5 speed? They are pretty well known to stall after a 5 speed conversion unless the ECU is modified by AJ6 Engineering.

But if that’s already the case… Stalling is low idle from bad AAV even if you replaced it, low idle from AC compressor coming on but supplementary air valve not opening, mal-adjusted TPS, mal-adjusted throttle plates, AC not working at all so the fuel cooler isn’t working….

We need more info I guess.

 

Adding to the "could be" list....

The ignition module and the coils can give trouble after absorbing a lot of engine heat

Cheers
DD

 

5spd conversion done several years back, so not due to ECU not being modified.

I am hazy on details, but conventional AAV replaced
by simple non-automated simplified manual adjustment screw. It does not auto adjust mixture based on engine temp. Its one single setting hot or cold. It was reset, and issue reoccurred under worse case conditions so thinking something else.

AC off entire time, but does work.

So possibly TPS or throttle plates?
I did read that a dodgy coolant temp sensor could be cause. It does occasionally stay on longer than usual after start up.

 

PO modded to single coil, if that matters.
Okay, something to consider......thanks!

 

My findings and fixes over too many years and HOT summers.

Some already mentioned.

1) The AAV has NOTHING to do with mixture control/variation. It is ONLY an Air Valve, allowing more air in for cold start. I dont run them on any of mine.
2) The CTS has NOTHING to do with the Low Coolant Light. That is a specific sensor in the steel header tank. The CTS is the Prime signal to the ECU, and the Vac line and TPS are the Prime trimmers. Sounds complicated, ITS NOT.
3) The integrity of the Vac line under the car to the ECU. It blocks up, cracks etc, so the ECU "sees" a dodgy signal and shuts down.
4) That shielded wire to Pin 18 of the ECU is shorting when it gets HOT. V12 engine bays are HOT.
5) The Hot Soak start Sensor fiasco on the fuel rail, oh dear, that is removed down here day 1 of ownership, then the associated wiring is tidied up.
6) TPS is out of adjustment, and/or flaky in that Lo Rev section, very common.
7) Throttle discs gummed up and set wronmg. I use 0.003" instead of the 0.002" in the book. Sounds small, but makes a huge difference.
8) The AB14 Amp on the B Bank Inlet manifold is getting way too hot. Move it to the front panel, where the 2nd coil once lived, and mount it on a good alloy plate for heat transfer, and space it up about 1/2" so air can flow ALL around it. I got snaps of mine somewhere if needed.
9) IF you still have the A Bank FPR, remove it. A PDF is available if needed.

I will keep thinking and return later, duty calls.

 

I am with Grant on this. I am sure this is an electrical/electronic fault and nothing to do with the tickover. My favourite is the amplifier giving up when hot. I would replace it with new anyway. Next, replace the CTS (coolant temp sensor) on the B bank forward water manifold (thermostat housing). These are cheap. Also replace the Air temp sensor on the B bank airbox inlet. Also cheap.
CAREFULLY check the wiring to these two sensors, unwrap the last foot or so and if it is crisp replace the part that is with silicone insulated wire and use a new connector (Bosch EV1). I recently did all this as preventative maintenance andI noticed that my hot starting was quicker to fire.
Moving the amp to the front is a great plan, I posted about this with pics about a week ago; BUT if the amp is OK the engine will not stall for no reason even in the OEM position, so I would cure this problem first and then, as a belt and braces measure, when you are sure, move the amp.
Final thought, there are two wires that exit the dizzy base and via a rubber connector and an extra bit of loom. This plugs into the side of the amp with a flat plug. This is hard to get at with the cruise in the way, but very carefully check this item to ensure it is not shorting/failing.

 

Gents,
Fantastic input as ever....... thanks!
A few things:
- Transistor in amplifier replaced 3yrs (5k km) ago with GM part (AC Delco ACD # D 1906 / GM # 19180771) as per Palm mod. However, I understand the relaibility / quality of these parts can vary widely, so will replace anyway as advised ($CAD75). Greg - that is what you are referring to by 'replace the amp', or did you mean the whole Lucas AB14 unit (more $), (DAC4104).
- Amplifier has already been relocated to the front rail.
- CTS - this is DAC4737 correct?

Will replace amp and 2x sensors as suggested as fix (hopefully), or PM. Will then go through other steps as kindly suggested.

All the best,

Quentin

 

Quentin
I have had no luck att all with just replacing the module. I bought a new entire amp and all good. Manners have them not too dear; buy the cheapest as the only difference is the plug to the white wire which you can easily change.
https://www.jagspares.co.uk/product/...-84/dac2673%23
Loads on ebay too.

 

For engines that suddenly stop when they get warm, carry a bottle of water. Open the bonnet, pour a bit of water over the ignition amp module for 30 seconds.
There is about 50% chance the engine will start again.

 

Got it,
Thanks!

 

Greg,
So a month or so back I replaced the amplifier part only. Seemed to cure it. Problem returned last week. Am now replacing the whole AB14 unit but am a bit confused by the wiring that goes to terminal C on the amp (-ve from coil). Below is the old module. Wires 1 to 3 are spliced together into one wire (4) that goes to C terminal. Wire 3 is an earth.

On the new AB14 unit there is wire 1 (earth) and wire 2 (C terminal). Now electronics ain't my forte, but is the new unit wiring correct?

 

Have replaced the air temp sensor. Cracked insulation. New connector and pigtail to be added (as per your suggestion).


I then looked at the coolant temp sensor (also as per your suggestion)........ there are two! I only bought the EV1 connector type replacement sensor. The coolanr temp EV1 connector is non-quick release (air temp sensor was quick release). I gave it a brief tug and no go, but bit hard to access. Do you have to release the wire retaining clips with these, or just yank off?

 

When it won't start, check for continuity across the distributor pickup. I had an '87 once with identical symptoms and the pickup coil was open circuiting when hot. It's getting even more baked than the amp being down there in the "V" within the distributor body.

 

Quote:

Do you have to release the wire retaining clips with these, or just yank off?

Release the wire clip. A small tipped pointy tool is useful here.

Cheers
DD

 

Thanks for your input guys. Always appreciated!