4AGZE bouncing idle and stalls when warm

[BIGBOBO]

I'm almost done with my NA to SC conversion but I can't figure out the timing. Every time I short the T and E1 connector the rpm drops so much the engine dies. I tried opening the idle adjustment screw but then the engine starts hunting.

The BGB suggests that to get 800 rpm idle you should close the idle adjustment screw all the way so I don't think opening it would fix the stalling problem.

I also tried shorting T and E1 with a cold engine but then it also starts hunting. The engine also stalls most of the time after I rev it a bit but this is probably due to the incorrect timing.

Does anyone have any idea what's going on?

[kaiowas]

What's your timing like without T and E1 bridged? Bridging the diagnostic pins switches the ECU from running it's normal variable advance to a much lower, fixed value. The fixed bit usually being the important bit as it allows you to set the timing to a known point when in this mode.

My suspicion is that your timing is so far out that when you bridge the connectors it's running so little advance that it can't sustain an idle.

Root cause is most likely the dizzy being a tooth out (unless it's a DLI engine)

[BIGBOBO]

I think the timing is pretty close to the right setting, I timed it while it was idling at 800 rpm (without shorting the connectors) and it ran, reved and started fine. Even drove it in and out the garage a few times.

I will check if the distrubutor is a tooth out but I'm pretty sure it isn't since I already had that happen on my NA engine and I took extra care inserting it on this engine.

[jon_st205]

IME if the dizzy is a tooth out the car will barely run at all, maybe a cylinder or two will just about fire.

Have a look at the TPS to make sure it is not off its idle switch (or has damage to the TPS plug/wires), this would cause the problem you describe.

[BIGBOBO]

Ok I'll check the TPS. I took appart and cleaned every part of the intake system when swapping the engine but left the TPS on since it was obvious you have to adjust it.

There was red paint on the screws of the TPS, not sure if these are stock alignment marks or someone has already been tampering with it.

[jon_st205]

Red will be a mechanic having set it in the past, worth testing it before adjusting though, especially the plug & wires.

[BIGBOBO]

I tested it today (after waiting 2 days for the shop to order some feeler gauges) and it slightly out of spec. The idle circuit should be closed at 0.65mm but isn't, it is at 0.7mm however.

I tried adjusting it but the screws holding the TPS are absolutely mangled. Managed to get one of them out but I have no idea how to get to the other one out, I probably have to take intake appart and find some new screws. They are probably fine threaded so that's gonna be a bother too.

Just want to get this car back on the road after almost 1.5 years

[jon_st205]

Yeah I had to remove the TB and drill mine out. The usual choice is re-tap the holes and fit some allen key bolts or similar.

[ulysess1966]

I've had the same problem. I set the TPS, got it wrong and had to take the whole throttle off again to get the screws out. I replaced them with 2 with hex heads to make life a bit easier.

[BIGBOBO]

I managed to get the screw out and adjust the TPS. To bad, the hardware store didn't have M4x12 hex bolts so I had to use screws again.

Unfortunately it didn't help my problem . I managed to set the timing by adjusting the idle way to high, shorting the terminals to go into diagnostic mode and lowering the idle extermely slowly. If I did it to fast it the idle would start to bounce or the engine would stop running.

The timing is spot on now but the car still doesn't idle properly. When starting it (hot engine) it bounces between 500 and 1500 rpm most of the time it stabilises itself (around 900-1000 rpm) but sometime it just dies. The same symptoms apear after revving it a bit, it always starts to bounce, stabilises itself most of the times and sometimes dies.

[jon_st205]

Next stop given those symptoms would be the water temp sensor, assuming you've bled the coolant. You can measure the resistance to see if it's in spec.

[BIGBOBO]

Coolant is bled and the water temp is within range (both cold and hot) but the problem stays the same

I tried disconecting the TPS, cold start injector and idle control to isolate the problem but none of them solved the issue.

I removed the AFM to clean later today, but don't think that will work, the flapper seemed to move freely.

Another thing I noticed it that when starting it from cold it sputters a bit before starting. It doesn't do this when the engine is warm.

UPDATE: someone told me my Idle Control Valve could be dirty, I'll take it out tonight and clean it.

[BIGBOBO]

Just cleaned the idle control valve but it didn't seem to help. I tested the control valve by applying power to it and it opens in both directions.

No clue left on what to try .

(Updated the topic to better reflect the issue).

[elbon50]

My towcar uses a MAF sensor (AFM)

If there's a problem with the MAF the engine will run noticably better with it disconnected

Never seen a Mk1 with MAF (only MAP)

Can you try disconnecting the MAF on yours ?

Also very unwise to try cleaning the MAF (buggers it permanently)

[BIGBOBO]

The early 4A-GZE uses an AFM. I didn't open the AFM only tested if the flapper moved freely and blew some brake cleaner through the bypass hole to see if it wasn't clogged.

[elbon50]

On my towcar I can easily disconnect the electric lead to the MAF

When it started to play up I disconnected it & the engine ran much better

Bought a new sensor & problem solved

I have a scanner for that car though, gives me pointers to problems

Have you tried checking for fault codes on yours ?

[ulysess1966]

This is probably a long shot, but the ECU uses 2 power transistors to drive the ISCV and it might be worth testing the output from the ECU to the ISCV. I did this by checking for voltage with the car running on both the 'open' and 'close' connection.

[BIGBOBO]

The car doesn't throws any error codes.

Can I measure the voltage by disconnecting the valve when it's running? Or how should I reach the cables otherwise?

I'm thinking I just have a very well hidden vacuum leak that's letting in more air than measured and leaning out the mixture. Or a VSV that's not functioning properly.

The search continues tonight!

[BIGBOBO]

I tried looking for a vacuum leak last night but the idle is just to unstable to find it using the traditional spray carb cleaner method. It will rise and lower on it's own even without touching anything.

Spraying carb cleaner on the same spot will lower the idle the first time, next time it will rise or stall or do nothing at all. It just seems random.

Not sure what to do anymore.

The engine is bone stock except for a Walbro 255 fuel pump. Not sure if this would cause the problem.

[BIGBOBO]

Took out the sparkplugs today and they were black, so it's running rich. Seems I also messed up while switching them last time. There were 2 different types in it. Replaced them all with new ones.

Cleaned up the ignotor and coil bracket cause they were rusty and I read they are a grounding point.

Unfortunately neither solved the problem :/.

So I started measuring the voltages of the ECU as described in the BGB and found one that is incorrect.

VTA to E2 should be 4 to 5 volt when throttle valve is fully opened. Was only 3.68. I already adjusted the TPS.

Any ideas?