The BBC understands that both companies are keen to sign a so-called "statement of intent" before the week Geneva Motor show starts on 6 March to prevent ongoing speculation overshadowing the release of new models.
Events that will determine the future of Vauxhall in the UK have been moving very quickly.
Since last week's surprise announcement that GM was considering a sale, senior management from both companies have met with politicians and unions in Germany and the UK. They fear that PSA, partially owned by the French government, may cut jobs outside France.
In a call with Prime Minister Theresa May last night, the PSA boss "expressed willingness to develop the iconic Vauxhall brand for its faithful customers". That's not quite the same as guaranteeing the future of 4,500 Vauxhall workers at Luton, Ellesmere Port and throughout the UK.
An outline deal will signal the start months of intense lobbying across Europe. Progress after any deal is signed will be slower.
It doesn't make much financial sense to stop production of a car before a new model is introduced and the Astra produced at Ellesmere Port is not due for an update till 2020. At that point, Ellesmere Port is considered particularly vulnerable to competition from a PSA plant in Sochaux in France.
The cost and complexity of moving car production at any point is formidable. Supply chains, logistics and IT platforms would require years of modifying.
For example, I'm told that at the Jaguar Land Rover plants in the UK, computers still briefly show a Ford prompt when users log in - and Ford sold JLR to Tata a decade ago.
Nevertheless, one thing all industry experts appear to agree on is that the 24 plants in Europe that a combined company would have is too many.
With only two factories of those 24 - and those plants soon to be outside the EU - the UK is considered the lightweight in this manufacturing tug-of-war.
The government here has stressed its support for the automotive sector with particular emphasis on new electric vehicles and battery technology - something considered key in convincing Nissan to promise increased investment in the UK.

Police were called to the woman's mobile home after receiving a 911 call about a woman suffering chest pains.
But when officers arrived, the woman pointed a gun at them and threatened to shoot anyone who came near her.
The woman stayed put despite police using flash grenades and tear gas.
The standoff in the town of Topanga, near Los Angeles, finally ended early on Friday after the woman attempted to crawl under her mobile home and police pulled her out.
"It's very unusual, but we're just happy that we're able to bring this to a peaceful resolution," said Lt. A J Rotella of the Sheriff's Department's Malibu station told the Los Angeles Times. "No-one wants to see an elderly woman, for whatever reason she acted this way, no one wanted to see her get seriously hurt."
It was unclear whether the crisis-negotiation robot was damaged in the incident. The woman also hit it with a broom and threw a cover over it.
Some police departments in the US use the robots - equipped with two-way radios and cameras - to scout dangerous situations and establish communication between police and suspects.

The £21m road, which will link the west and east of the town, was supposed to be finished by March.
However, last winter's heavy rain meant contractors could not start building a bridge over the River Tone and the Bridgwater and Taunton Canal.
Richard Needs, from Somerset County Council, said it had also encountered other challenges during the project.
He said work to lift an existing bridge over Station Road had to be delayed as workers had found the ground conditions softer than expected.
"It's not a question of moving the bridge, it's working out what are the best foundations for the new bridge to be," Mr Needs said.
"As with all construction projects there are very difficult times and there are happy times... it changes from day-to-day but overall we're making good progress and we're getting there eventually."
The one-mile-long (1.6km) road is designed to divert traffic around the north of the town and will run from Priory Bridge Road, over the river and canal, alongside the railway to Staplegrove Road.
It is hoped the new road will divert traffic away from the town centre and Priorswood Road and help with the regeneration of the Firepool area.
A draft agreement to sell General Motors' European business to PSA, the owner of Peugeot and Citroen, could be signed as early as next week.
[NEXT_CONCEPT]
An elderly California woman has surrendered to police after a 22-hour stand-off, in which the woman shot at a police robot that was sent to communicate with her.
[NEXT_CONCEPT]
The completion date for a new road in Taunton has been delayed.