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Matt, Engineer
Category: Ford
Satisfied Customers: 21858
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have a 1993 Ford Thunderbird 3.8L SC Supercharger (bypass

Customer Question

I have a 1993 Ford Thunderbird 3.8L SC Supercharger (bypass ignition system/waste spark design) that has a soft fault while running and sometimes at idle. The car bucks and jerks both at idle (sometimes) and while driving. There are codes that are present, memory code 221 Spark Timing Error (base timing checked good) and a code 121 Closed throttle voltage higher or lower than expected (TPS checked good no glitches, shorts drop outs and set at proper voltage 1V at idle). During fault engine vacuum not affected.
I own a Snap On Vantage PGM and conducted several of tests (volts and amps) while connected to inputs and outputs while the fault was occurring and found nothing yet to be at fault. I'm thinking I may need some known good wave-forms for signal alignment. No parts have been replaced. Please help
Submitted: 2 years ago.
Category: Ford
Expert:  Matt replied 2 years ago.


This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile

Customer: replied 2 years ago.
Hey Matt,I scope tested the MAF sensor and found to be within range no glitches no dropouts no shorts or opens good power and ground during the fault. Remember I'm driving while connected to these sensors and I'm looking for signal problems while the fault is occurring.Fuel pressure during the fault was within spec and never changed during the fault (32 psi). What do you think about the codes? I'm surprised you never mentioned cam crank or other signal input problems yet. This also feels like a large misfire chugging intermittently as well.
Customer: replied 2 years ago.
Matt,Are you a member of IATN? If so, are there wave-forms you can give me to complete my diagnosis?
Expert:  Matt replied 2 years ago.


thanks for the extra information

I don't think there's a cam sensor on these, as its wasted spark as you say and just batch fired injection ( so LH bank then RH bank)

and as the fuel pressure is OK I wonder if its worth doing a compression test across all cylinders to ensure that all cylinders are within 10-15 psi of each other

Customer: replied 2 years ago.
Matt there is a cam sensor on this design. I have all the diagrams, again this is a bypass ignition system with a cam and crank into ign. control module and PCM, SPOUT wire from PCM to ign. control module for spark timing advance. All is there and working properly no glitches no dropouts no shorts good power and grounds. You mentioned about compression it would be hard to do because mostly when the car acts up it's during driving and it would be hard to do a running compression test without a compression transducer. I checked for vacuum leaks like you said and found none. Are you a member of IATN?
Expert:  Matt replied 2 years ago.


no I'm not a member of IATN, as I'm in the UK

what you describe certainly sounds like a loss of ignition or fuel to me

and while I appreciate what you're saying about there being no loss of signals , electrical faults won't always show up as the ECU can send signals out with no feedback, Injector signals for example have no feedback into the ECU as its too old an system for that

So I'd suggest doing some basic testing

Check theresistance of the spark plug leads ( 13-15Kohm / metre) and replace if anydoubt or high resistance - Also replace the spark plugs if any of them failthis checklist:

You can also try spraying some water on the coil pack with the engine running. It is possible the coil is faulty and causing the misfire. If you see sparks arcing with the water, then replace the coil.

I'd check the condition of the injector plugs / connections as this could be a lean misfire if an injector isn't firing properly - the best way to check this is with a 'noid' light

Customer: replied 2 years ago.
Hey Matt,
The spark plugs and wires were changed years ago. This car is only driven during the summer months. So basically the plugs and wires have less than 5,000 miles on them therefore, I doubt this is a secondary ignition fault. Just as I said before I checked the primary ignition side (coil ramp test) all coils have normal current flow, good triggering and inputs from module. Also I current ramped injectors and checked pulse width while the fault occurred and found nothing to be wrong. I will spray down coil pack with water and check for fault.In order for me to complete my electrical troubleshooting I would need some waveforms, are there any people on here that can supply certain known good waveforms to me?
Expert:  Matt replied 2 years ago.

thats possible, I'll opt out and try and get someone else involved

good luck

Customer: replied 2 years ago.
Okay then, I'll be looking forward to the new expert. Thanks for your help anyway Matt.
Customer: replied 2 years ago.
Is anybody going to help me? It's past the 24 hour mark!