Engine hesitation at half throttle - likely cause?

[jim290s]

Hi all, I've had my S2 for just over a year now, and all has been going well(ish). As luck would have it, the car has started to develop a fault with a couple of weeks to go before the S Club Euro Tour... and it's going to drive me crazy if I can't figure out what the problem is before then.

The car starts and idles perfectly, always first try from cold. Engine sounds good, and doesn't sound like it's running rough on tickover.
The problem is, the engine can hesitate between about 1/4 to 3/4 throttle load. So when my foot is flat to the floor, or with very light throttle load - no problem. However, if gradually accelerating with 1/2 throttle load, the power comes through in little surges. Feels like the car is jolting.

The car seems to have developed this fault gradually, and it doesn't happen on every drive. Sometimes I can drive around for an hour or two with no sign of acceleration issues, and then on other drives it's getting really quite bad.

As the Euro tour is pretty close now, does anyone have some easy things to check first? Or has anyone else experienced the same problem?
Car is a standard 1989 with EFI/injection etc. New plugs fitted a few months ago. Nothing else fiddled with.
Any help would be appreciated! Thanks

[jim290s]

Ok, so after trawling this forum and others for a little longer, it appears that the throttle position sensor dies fairly regularly on the S series, and seems to match the symptoms i'm experiencing. Unless anyone else has any other ideas, it sounds like getting a new one on order is the way to go?

[ATS3]

For what it's worth it sounds like the TPS to me.

Andy

[v8s4me]

When was the last time you changed the fuel filter?

[jim290s]

I've not changed the fuel filter since I bought the car, as the service interval is quite long for the Sierra, and the history stated it had one fairly recently. I've only done a couple of thousand miles. I've bought a replacement filter anyway, but just not got round to fitting it yet.

I'll try the TPS disconnection method to get into 'safe' mode and see if that solves the issue. Probably get the filter changed now anyway just to be on the safe side.
Taa both

[S2Andy]

I would check for air leaks. Particularly the base of the brake servo and its pipe to the plenum. Also check the air flow meter flaps are free and clean the little wires inside, and check the condition of the two air pipes to the throttle. Finally are your plug leads OK. Just changed mine and made the engine revs much more cleanly. Andy. PS See you on Euro Tour.

Edited by S2Andy on Tuesday 1st September 08:05

[swimmerS2]

Just in case it helps during my S2 ownership the following caused me similar problems.

1. Fuel tank had so much muck ie sludge, rust etc it was restricting the flow of fuel.
2. The two AFM connectors had corrosion on the tiny pins
3. The ignition coil failed causing not only hesitation but also the odd backfire

[keith-vznby]

Just had exactly the same problem it was the TPS

[jim290s]

Ok, so I checked for leaks this eve, checked all connected pipework and couldn't find anything obviously perished. Disconnected the TPS and wow!
I'm starting to think that the TPS might have been iffy for the entire time I've owned the car - it was roughly 30% faster in acceleration than I think it's ever been!
So now to choose a new TPS. There a loads on ebay for the 2.9i Sierra, but lots of different codes.
The one's I'm looking at are described as:
LEMARK LTP012 THROTTLE POSITION SENSORS FOR FORD SIERRA 2.9 1988-1993
Are all of these TPS sensors the same? The thing which is confusing me is the extra code: 3690, 3702, 3703, 3704, 3705, 3708, 3733, 3734, 3735, 3736, or 3737!
All of these are stated to be for the 2.9 Sierra (88-93) but which one is right for a 1989 TVR S2? Or are they all the same base part regardless of the description on ebay? The price is the identical for all 11 listed.
Thanks for the other fault suggestions, good to check everything else over, but glad it looks like I've found the culprit

[TopVpowerRoadste]

What is a TPS and where can I find it in the engine bay?
Maybe worth checking.

Many thanks,

Geoffrey

[phillpot]

No idea why so many part numbers but wasn't there an issue with some having silver connection pins and some gold, if you change you need to swap the wires around?


This is a good read.... clicky and there is plenty more with a bit of searching



TPS's can be tested, there should be 0.6volts at the centre (green?) wire when closed, rising smoothly to 4.5 volts when fully open. tricky bit is connecting your meter, I stabbed the wire with a pin and clipped test lead onto that

Edited by phillpot on Wednesday 2nd September 09:55

[keith-vznby]

If you take out the two screws the ford part number is on the unit quote this and any motorfactor will be able to cross reference that's all I did no probs.The TPS is on the oppositeside to your throttle cable and is on fact activated by throttle butterfly spindle heid on by two bolts 7mm I think can be changed in 5 minutes.After changing take the car out and run it up to temperature to let the system reset itself cheers Keith

[TVRees]

Check the Alternative Parts List also, which shows this info:

• Throttle Potentiometer. Ford Motorcraft 6854780 or Intermotor 19935. Now with gold connectors : http://partfinder.smpeurope.com

• Throttle Potentiometer Wiring Assembly. Ford Motorcraft 5028499. Required to rewire the above.

[TVRees]

TopVpowerRoadste said:

What is a TPS ...

Here is some interesting reading
engine sensors

[lewdon]

I had a similar problem. the Throttle Position Sensor is so cheap and easy to change it should be the first thing you try. However after I replaced the TPS the problem re-occurred. Eventually cured it by cleaning the tracks in both the air flow meters and switching the plugs from the top to the bottom AFM and vice versa.
Good luck

[jim290s]

Thanks. I've taken the fitted TPS off the car this eve, and the code stamped into it is E1586. It also has a Ford stamp.
Looking at other threads, some people have mentioned that E1586 is the Cerbera 4.2 TPS code, and that the original Ford part has the code 90TF9B989.
Very confused about which one to order now, especially as this eBay listing contains both the 90TF9B989 and E1586 codes. link
I don't really want the Cerbera TPS as others on this forum have had problems with throttle response. Hopefully the TPS in the link above is the right one for my S

[lewdon]

My 1989 S2 2.9 original throttle pot was marked 86TF-9B989-AD. my local factor came up with an equivalent to Intermotor 19935. It was three years ago but my recollection is that it fitted without any modification though I recall had to use the original metal strip as the new TPS came without one. I think it cost me about £25. I hope this helps.

[TVRees]

According to this .... #6, fordopedia

The 90TF 9B989 is for later model engines.

[jim290s]

Very useful info. The eBay item I linked to in my earlier post also contains the 86TF-9B989-AD code though. Obviously the parts can be interchangeable, but I'd like to get the 'right' one for my engine. I'll have a look specifically for a part only specifying the 86TF code I think.