1993 2.0L manual wentworth turbo UK

Picked up a new problem yesterday where the car started running a little rough at lower speeds but just managed to limp home. Seemed to be having trouble with ticking over.

Left overnight and cleaned iac controller this morning (it was very clean surprisingly) and checked vacuum hoses (all good bar one which I replaced)

On startup, thought it was better till I blipped the throttle then it started surging the tickover up and down all on its own! I stood and watched this for about 8 cycles, noting that it dropped the revs almost to cut out then brought them up again to about 1200 revs. The only visual fault I can see under the bonnet is a crack on the distributor cap where the top mounting screw goes (i can see the metal but theres no arcing going on) and the usual audible clack clack clack from the leaking exhaust port gasket ive been meaning to fix for years. (ive now bought the gaskets)

Also, I just tried a different Fuel pump relay which made no difference to the behaviour, but I did note interestingly that on startup the engine starts and idles at about 700rpm absolutely fine for about 20 seconds then starts hunting up and down.

I would be most grateful for any pointers on how to get going on this mystery. Im used to it being a bit of a struggle but any shortcuts would be very welcome.

regards

Dear Pierso,

Hope you're well and stay so! The first thing to check is the Idle Air Control Valve, mounted on a bracket beneath/beside the intake manifold.

Remove the valve and inspect its openings. A dust/dirt build-up can disrupt the valve's operation. Put brake cleaner on cotton-tipped swabs and carefully wipe dirt from valve openings. Have a tweezers handy to remove cotton fibers that separate from the swab's tip.

Keep the valve's end with the electrical connector pointed upwards: this keeps solvent from entering the motor compartment.

Before re-installing, use an electrical oxidation remover on the valve's contacts and on the wiring harness contacts. Deoxit has served me well.

With the valve cleaned mechanically and electrically, reinstall it. I'd guess this should improve things.

You should also inspect the large-diameter air-intake hose, that runs from the air box to the intake manifold. A crack in the corrugated section of this hose can allow unmetered air to enter. If the crack is large enough, the amount of unmetered air may exceeded the computer's corrective capacity.

Hope this helps.

Yours faithfully,

Spook

Thanks Spook, good advice as always. I tried my back-up carefully cleaned original bosch iac controller and it was no different unfortunately. I had a pretty good look around the hoses as you suggested but no smoking gun really. I'm possibly going to try a new fuel pressure regulator with a some new vac hose as the next step. The car's been so good over the years it probably deserves some new parts even if they aren't the answer! I certainly think the poor old computer is having a hard time of making sense of whatever the problem is.

Will update this thread in a couple of days when the regulator goes on.

regards

Thanks Spook, good advice as always. I tried my back-up carefully cleaned original bosch iac controller and it was no different unfortunately. I had a pretty good look around the hoses as you suggested but no smoking gun really. I'm possibly going to try a new fuel pressure regulator with a some new vac hose as the next step. The car's been so good over the years it probably deserves some new parts even if they aren't the answer! I certainly think the poor old computer is having a hard time of making sense of whatever the problem is.

Will update this thread in a couple of days when the regulator goes on.

regards

Thanks Spook, good advice as always. I tried my back-up carefully cleaned original bosch iac controller and it was no different unfortunately. I had a pretty good look around the hoses as you suggested but no smoking gun really. I'm possibly going to try a new fuel pressure regulator with a some new vac hose as the next step. The car's been so good over the years it probably deserves some new parts even if they aren't the answer! I certainly think the poor old computer is having a hard time of making sense of whatever the problem is.

Will update this thread in a couple of days when the regulator goes on.

regards

I've seen this happen with a failed fuel pressure regulator. Excessively high pressure was causing fuel to leak from the cold start injector. The extra fuel raised the rpm, ecu cut fuel or ignition (both?) to prevent overrev, engine nearly stalls, whatever the ecu cut is then restored, rinse and repeat.

Thanks Allrounderco,
Looks like what you described fits the symptoms I have so its worth a try.
I ordered one today with some vac hose. No harm done if its not that, it'll take the crown for best new part on my car!

regards
piers

Hi Pierso,

Check for OBD codes at ports A2 & A6
See if you get a 2-1-4 at A6 (hunch)

Start by replacing your cracked distributor cap.
That will raise hell in several ways.

Post back with some good news,

Bill

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Thanks Bill, got a 1-1-1 on port A2 and 1-1-1 on A6. Maybe that cap is cracked in more than one place. Have looked out my spare distributor cap and will try that tomorrow.

Hi Pierso,

Get rid of the cracked distributor cap, it's poison.

See what happens afterwards.

Bill

Changed the distributor cap and rotor arm

What a nasty little job that was (fractured my wrist last week so a bit of a challenge).
Admittedly the cap i took off was shocking, but that hasnt sorted my problem unfortunately.
Tried swapping out the iac controller too, but that isnt it.
Rechecked hoses for holes and splits - all ok.

I cant get over that it starts, ticks over for 10-15 secs, then drops the revs then brings them up to 1.5-2k revs then down again! I got it do that on its own 8 times yesterday while I stood and watched.

Grateful for any thoughts .....

regards
piers

With an unstable idle like that and revs going up to 1,500 then down to a stall, there are only a few thoughts in my mind. You also need to start using the OBD system to help rule out a few possibilities:

a) The ECU doesn't even know it's supposed to be maintaining idle. This would by far be my #1 suspicion based on some of the things you've said. The TPS switch is either incorrectly adjusted, is faulty or the throttle plate is binding in the throat. This needs be verified using a strip of 24 bond paper as a spacer at the throttle plate stop screw, both to get a click and then to verify it with a meter. Double check that the throttle plate isn't binding (force it closed by jand to see if that helps), the throat is clean (especially around the edge of the plate, requiring the air supply tube to be removed) and that the TPS electrical connector is firmly seated. Much better and even simpler than using a meter to verify TPS operation and adjustment is to get the ECU to tell you if it can see the switch opening and closing. You should use the OBD system to verify this using DTM 2 on port 2 (as described here in the FAQ).

b) The ECU can't maintain idle. This would normally suggest a dirty/worn/sticky idle air valve or a loose connector. You said you replaced the IAC controller. I'm not sure what you mean by that. Again, use the OBD system to verify operation of the IAC using DTM 3 on port 2 (as in the FAQ).

c) The IAC can only adjust idle when the throttle plate is sufficiently closed when at rest. If the throttle plate is binding or there's even the remotest chance someone has ever played with the idle stop screw, the turnbuckle linkage rod or throttle cable then you need to disconnect/loosen and readjust everything, including the TPS, using the 1/4 turn method for the idle stop screw and 24 bond paper to adjust for the TPS click, making sure when you later connect it that there's a bit of slack in both the linkage rod and the throttle cable when the throttle is closed. All that should be in the FAQ.

d) There's an incorrect sensor signal, specifically the block temp sensor. Normally when this happens the idle goes high as it warms up and stays high, not going back down again. Still, I'd want to verify the ohms on both a cold and warm block. Resistance values can be found if you do a bit of searching here and elsewhere.

e) There's uncontrolled air getting into the throttle plate bypass air supply -the two hoses on the IAC. Check carefully for splits in those hoses, especially the one that goes to the throttle body.

--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thanks Dave for that excellent advice, I will systematically go through the items youve listed as they all make good sense. Its the suddenness of the problem that gets me which makes me think component failure is the reason, though something like the tps or a catchy throttle could well have reached a failure point, also I know i havent touched any of that part of the system since i did the head gasket and heater matrix years ago.

I'm going to try bunging on a new fuel pressure regulator and if that doesnt work a miracle I shall go through things properly.

(I got 1-1-1 on port 2 and 1-1-1 on port 6)

many thanks
piers

Tried replacing the Fuel pressure regular but this was unsuccessful. Then tried a new iac controller - also unsuccessful, then I cleaned the MAF, also unsuccessful so this led me to check all the vac hoses by taking them off and making sure nothing had collapsed inside by poking something down them. Acting on intuition, I decided to see what the car ran like if I pulled the 22mm pipe off the plastic y pipe that carries the small vac tube to the inlet manifold . This links onto the crankcase emulsifier unit and happily for whatever reason the engine ran fine with this off. I also found that the pipe that comes across from the MAF/turbo pipework to the crankcase emulsifier had cracked half way along (its a plastic pipe with a foam insulation sleeve on it so you'd never know it had cracked). I mended this pipe, cleaned out the emulsifier unit with petrol first then 40 minutes in an ultrasonic bath at 50 degrees with some detergent and put it all back together again.

I dont really understand why the engine behaved like it did, it doesnt have a flame trap and I couldnt really feel a vac or a pressurisation of this pipe when I capped it with my finger. Ive read that flame traps sometimes give grief but mine doesnt have one being the turbo. Interested if anyone has any thoughts.

regards
pierso